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STUDIES CONCERNING THE ELIMINATION 
OF EXPERIMENTAL ERROR IN COM- 
PARATIVE CROP TESTS 



By T. A. KlESSELBACH 



A THESIS 

PRESENTED TO THE FACULTY OF 

THE GRADUATE COLLEGE IN THE UNIVERSITY OF NEBRASKA 

IN PARTIAL FULFILLMENT OF REQUIREMENTS 

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY 

DEPARTMENT OF BOTANY 



LINCOLN, NEBRASKA 
December, 1917 



Reprint from Nebraska Agricultural Experiment Station 
Research Bulletin No. 13 



STUDIES CONCERNING THE ELIMINATION 
OF EXPERIMENTAL ERROR IN COM- 
PARATIVE CROP TESTS 



By T. A. Kiesselbach 



A THESIS 

PRESENTED TO THE FACULTY OF 

THE GRADUATE COLLEGE IN THE UNIVERSITY OF NEBRASKA 

IN PARTIAL FULFILLMENT OF REQUIREMENTS 

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY 

DEPARTMENT OF BOTANY 



LINCOLN, NEBRASKA 
December, 1917 



Reprint from Nebraska Agricultural Experiment Station 
Research Bulletin No. 13 



G^w £ 



AGRICULTURAL EXPERIMENT STATION OF NEBRASKA 



THE GOVERNING BOARD 

(the regents of the university) 

HONORABLE E. P. BROWN, President, Davey. 
HONORABLE J. E. MILLER, Vice President, Lincoln. 
HONORABLE FRANK L. HALLER, Omaha. 
HONORABLE VICTOR G. LYFORD, Falls City. 
HONORABLE PHILIP L. HALL, Lincoln. 
HONORABLE HARRY D. LANDIS, Seward. 



THE STATION OFFICERS 

*SAMUEL AVERY, Ph. D., LL. D., Chancellor, ex officio. 

W. G. HASTINGS, A. B., Acting Chancellor. 

E. A. BURNETT, D. Sc, Director. 

C. W. PUGSLEY, B. Sc, Director of Extension Service. 

J. S. DALES, M. Ph., Financial Secretary. 

C. A. LEWIS, B. Sc, Bulletin Editor. 



THE WORKING STAFF 

F. W. Upson, Ph. D., Agricultural Chemistry. 

*L. W. Chase, M. E., A. E., Agricultural Engineering. 

W. W. Burr, B. Sc, Agronomy. 

T. A. Kiesselbach, A. M., Agronomy. 

H. J. Gramlich, B. Sc, Animal Husbandry. 

J. H. Gain, M. D. C, Animal Pathology. 

J. H. Frandsen, M. S. A., Dairy Husbandry. 

Lawrence Bruner, B. Sc., Entomology. 

M. H. Swenk, A. M., Entomology. 

H. C. Filley, A. M., Farm Management. 

R. F. Howard, A. M., Horticulture. 

G. A. Loveland, A. M., Meteorology. 

E. Mead Wilcox, Ph. D., Plant Pathology and Physiology. 

F. E. Mussehl, B. Sc, Poultry. 

W. P. Snyder, M. S., Superintendent Experiment Substation, North Platte. 

James Cowan, M. E., Superintendent Experimental Substation. Valentine. 

James A. Holden, B. Sc, Superintendent Experimental Substation, Mitchell. 

tL. L. Zook, B. Sc, Agronomist, North Platte. 

*J. W. Calvin, B. Sc, Associate in Agricultural Chemistry. 

tG. K. K. Link, Ph. D., Associate in Plant Pathology and Physiology. 

Erwin Hopt, B. Sc, Assistant in Agronomy. 

W. B. Nevens, M. S., Assistant in Dairy Husbandry. 

R. R. Spafford, A. M., Assistant in Farm Management. 



*Granted leave of absence for military service. 

tDetailed from Office of Dry Land Agriculture, United States Depart- 
ment of Agriculture, Washington, D. C. 
tGranted leave of absence. 



tUG 2/. 



*mo 



CONTENTS 

Page 

Summary 5 

Error due to competition between adjacent plats 14 

Illustration of principle of competition between adjacent rows 14 

Competition between adjacent rows of small grain 14 

Row competition in rate-of-planting tests with wheat and 

oats 15 

Relative stooling of two rates of planting when compared 

in alternating rows and alternating blocks 18 

Row competition between varieties of wheat and oats 19 

Evidence of plat competition in a wheat-breeding nursery 2 5 

Competition between individual plants 2 7 

Competition between corn test plats as a source of experi- 
mental error 2 9 

Row competition in rate-of-planting tests with corn 31 

Intra-hill and row competition in corn variety yield tests 33 

Summary of plat competition studies 41 

Variation of stand as a source of error in yield tests with corn 41 

Relation of stand to yield in single-row test plats 4 3 

Combination of rate planting with variety yield tests 45 

Effect of removing suckers with different varieties 4 7 

Reliability of estimating plat yields by means of fractional areas... 4 7 

Experimental errors caused by soil variation 51 

Use of check plats 5 3 

Reduction of error by replication 61 

Effect of size and shape of plat 64 

Significance of the probable error 6 5 

Probable error for 5 groups of four adjacent thirtieth-acre 

plats of Kherson oats 67 

Probable error of 5 groups of four systematically distrib- 
uted thirtieth-acre plats of Kherson oats 72 

Examples of limitation of the probable error 72 

Effect of change in methods on agronomic equipment 7 4 

Measuring improvement in yield thru breeding 7 5 

Soil limitation as a source of error in pot experiments 78 

Effect of the size of pot upon the growth of corn 80 

Effect of planting at different rates upon the growth of corn in 

pots _ 8 5 

Statement of methods in bulletins 8 9 

Bibliography 91 



The author wishes to take this opportunity to thank 
Professors R. J. Pool and W. W. Burr for the interest and 
advice they have given him in the preparation of this bulletin 
as a thesis for the degree of Doctor of Philosophy. 



SUMMARY 

1. In determining the effect of competition between 
single-row test plats as a source of experimental error in 
crop yield tests, the relative yields of two crops planted in 
blocks containing several rows have been regarded as the 
true relative values for the crops tested. In ascertaining 
some of these true values, the outer rows of the plats have 
been discarded in order to eliminate almost entirely plat com- 
petition. Plats were sufficiently replicated to secure quite 
reliable relative yields for the conditions under which they 
were grown. 

In plat competition tests in 1913 with two rates of plant- 
ing Turkey Red wheat, the thin rate yielded 68 per cent as 
much as the thick rate when grown in single alternating 
rows, while in five-row blocks the thin rate yielded 90 per 
cent as much as the thick rate. Competition in rows with a 
thicker rate of planting caused the. thin rate to yield rela- 
tively 24.4 per cent too low. In a similar test in 1914 the 
thin rate yielded relatively 56.8 per cent too low. 

2. In 1913, competition between alternating rows of two 
rates of planting with Kherson oats caused the thin rate to 
yield relatively 20 per cent too low. In 1914, similar single- 
row competition caused the thin rate to yield relatively 34.3 
per cent too low. 

3. In 1914, competition between alternating single-row 
plats of Turkey Red wheat sown at two rates reduced the 
relative number of stools per plant approximately 37 per cent 
for the thin rate. There was a similar reduction of 20 per 
cent for Kherson oats, due to plat competition. 

4. The relative competitive effect of varieties varies in 
different years, due to difference in adaptation to the seasonal 
conditions. 

In 1913, competition with Turkey Red winter wheat in 
single rows caused Big Frame winter wheat to yield rela- 
tively 10.3 per cent too high. In similar competition in 1914 
Big Frame yielded relatively 12.4 per cent too low. 

In 1913 there was practically no competitive effect be- 
tween alternating rows of Turkey Red and Nebraska No. 28 
winter wheat varieties. This was due to abnormal climatic 
conditions. However, in 1914 under rather normal condi- 
tions competition between single-row plats caused the Ne- 
braska No. 28 to yield relatively 25.9 per cent too low. 



6 Nebraska Agricultural Exp. Station, Research Bui. 13 

5. In 1913 in alternating single-row test plats of Burt 
and Kherson oats, the Burt yielded relatively 16 per cent too 
high, while in 1914 the yield was relatively 37.6 per cent too 
high, due to plat competition. 

In 1913, competition with Kherson oats in alternating one- 
row plats caused Swedish Select oats to yield relatively 7 
per cent too high, while in 1914 its yield was relatively 4.3 
per cent too low. 

6. When large and small seeds of wheat were planted in 
competition in the same row, the small seed, as a result of 
competition, yielded relatively 15 per cent too little grain, 
20 per cent too little straw, and made 18 per cent too small 
total yield. 

Similar competition was found between varieties of wheat 
planted in the same row. 

7. In a single-row test of 80 strains of Turkey Red 
wheat grown in the same order each of four years, there are 
evidences of plat competition between strains. As an aver- 
age for four years, the poorest strain, No. 75, grew between 
strains No. 74 and No. 76, ranking one and five. A special 
test of these three strains in 1915 and 1916 disclosed that 
strains No. 74 and No. 76 were favored 20 and 15 per cent 
respectively thru competition with a less vigorous strain. 

8. In a rate-of-planting test with Nebraska White Prize 
corn, — in which two rates of planting, namely two and four 
plants per hill, were compared in alternating single row plats, 
— the thin rate yielded relatively 29.3 per cent too low in 
1914 because of plat competition. In 1915 the thin rate 
yielded 9 per cent too low because of plat competition. In 
1916 such competition caused the thin rate to yield relatively 
16.1 per cent too low. 

9. A large, medium, and small variety of corn were 
grown in plat competition studies during 1912 and 1914. 
These varieties were Hogue's Yellow Dent, University No. 3, 
and Pride of the North, respectively. In 1912, Pride of the 
North yielded 85 per cent as much as Hogue's Yellow Dent 
in alternating three-row plats, while it yielded 66 per cent 
as much in alternating single rows. When compared in the 
same hill by the intra-hill method, the Pride of the North 
yielded only 47 per cent as much as Hogue's Yellow Dent. 
Due to competition, the Pride of the North yielded relatively 
44.7 per cent too low when compared in the same hill, and 
22 per cent too low in alternating one-row plats. 



Experimental Error in Crop. Tests 7 

10. In 1914, due to plat competition, Pride of the North 
corn yielded relatively 51 per cent too low when compared 
with Hogue's Yellow Dent in the same hill, while in alter- 
nating single-row plats it yielded relatively 28.3 per cent too 
low. 

In a comparison of University No. 3 with Hogue's Yellow 
Dent, the University No. 3 yielded relatively eight per cent 
too low in single-row plats, and within the hill it yielded 
relatively one per cent too high. The lack of competition 
within the hill in this case may have been due to there being 
only two plants of a rather similar type in a hill. When all 
three varieties were compared in the same hill, the relative 
yields for Hogue's Yellow Dent, University No. 3, and Pride 
of the North were respectively 100, 96, and 28, as compared 
with 100, 98, and 53 in the center row of three-row plats and 
100, 98, and 38 in single rows. 

11. In 1916, inbred Hogue's Yellow Dent corn which had 
been greatly reduced in vigor by five years of self-fertiliza- 
tion was compared with the more vigorous first generation 
hybrid of two such pure lines, in blocks, rows, and hills. Be- 
cause of competition with the larger plants in the same hill, 
the inbred corn yielded relatively 44 per cent too low, while 
in alternating single rows, it yielded relatively 16 per cent 
too low. 

12. Studies with oats, wheat, and corn suggest that the 
yield of the border rows of narrow, adjacent test plats may be 
materially affected by plat competition. 

13. \yiien surrounded by corn hills having a full stand 
of three plants, two-plant hills and three-plant hills respec- 
tively yielded 10.5 per cent and 35 per cent more than a one- 
plant hill in 1914. In a similar test in 1917, two-plant hills 
and three-plant hills respectively yielded 67 and 102 per cent 
more than a one-plant hill. 

14. The average grain yield of a three-plant corn hill sur- 
rounded by a full normal stand of three plants per hill was 
465.8 grams in 1914. This yield per hill was increased 2.7, 
5.3, 13.1, and 43.1 per cent by the presence, respectively, of 
(1) one adjacent hill with two plants, (2) one adjacent hill 
with one plant, (3) one adjacent blank hill, and (4) two 
adjacent blank hills. In 1917 corresponding adjacent imper- 
fect hills increased the grain yield of three-plant hills, other- 
wise surrounded by a full stand, respectively 2, 9, 15, and 
25 per cent. 



8 Nebraska Agricultural Exp. Station, Research Bui. 13 

15. Regarding three plants per hill as a perfect stand, 
the reduction in yield of corn was not proportional to a reduc- 
tion in stand. With single-row plats, stands averaging 92.8, 
87.2, 82.7, 77.8, 73.1, 66.6, and 43.0 per cent yielded respec- 
tively 85.5, 88.1, 83.5, 82.2, 77.9, 74.8, and 56.7 bushels per 
acre. 

16. Satisfactory yield correction for corn based upon 
per cent of stand cannot be made, because the effect upon 
yield depends upon the distribution of the missing plants 
and because the effect upon yield is not proportional to the 
per cent stand. Comparable yield tests of similar varieties 
or strains of corn may be secured by basing the yield upon 
a counted number of hills containing a uniform number of 
plants and surrounded by a full stand. 

17. Corn varieties or types differing markedly in growth 
characteristics should be tested at several rates of planting, 
because the optimum rate for one is not necessarily that for 
another. Thus, as an average for two years, Pride of the 
North and Calico produced their maximum yield when grown 
at the rate of five plants per hill, while Mammonth White 
Pearl yielded best at the three-rate. In 1914, Pride of the 
North yielded most at the five-rate, University No. 3 did 
equally well at the two and three-rate, while Hogue's Yellow 
Dent produced best at the two-rate. 

18. The removal of suckers affects the yield of varieties 
differently, and for this reason suckers should for no reason 
be removed in comparative variety tests. 

19. In comparative yield tests where it is not conven- 
ient to harvest and thresh the entire plats, fairly reliable 
results may be obtained by harvesting and averaging a large 
number of systematically distributed small fractional areas 
or quadrates from each plat. The necessary number of 
quadrates to be representative will vary with the size of the 
plats. 

Twenty 32-inch quadrates harvested from thirtieth-acre 
wheat plats gave fairly reliable results. Less than 20 proved 
likely to be unrepresentative of the plats. Very satisfactory 
results were obtained by having 40 quadrates represent one- 
fifteenth acre of wheat. 

20. Two hundred and seven thirtieth-acre plats were 
grown to a uniform crop of Kherson oats for the purpose 
of studying various phases of experimental error. Calcula- 
tions have been made from them to show: (1) The use and 



Experimental Error in Crop Tests 9 

effectiveness of check plats for reducing test plats to com- 
parable yields; (2) the reduction of error by the replication 
of plats; (3) the relative reliability of plats of various sizes 
and shapes; and (4) the significance of the "probable error" 
as a measure of confidence which may be placed in mean 
results. 

When the odd and even numbered plats of these 207 are 
regarded as check plats and test plats respectively and the 
grain yield of each test plat is corrected by the mean of the 
two adjacent check plats, the coefficient of variability for the 
actual yields of these test plats is reduced from 7.85 per cent 
for the actual yields to 7.01 per cent for the corrected yields. 
Assuming every third plat to be a check, and correcting the 
intervening plats by the one adjacent check plat, the coef- 
ficient of variability was reduced from 7.79 per cent to 7.35 
per cent. 

With every third plat regarded as a check plat, and the 
intervening plats corrected progressively by the two near- 
est checks, the coefficient of variability is reduced from 7.87 
to 6.57 per cent. Thus it is seen that none of the three 
methods of check plat correction have been very effective. 

The yield of systematically distributed check plats can- 
not be regarded as a reliable measure for correcting and es- 
tablishing correct theoretical or normal yields for the inter- 
vening plats. 

21. Systematic replication of plats is the most effective 
and satisfactory means for reducing error caused by soil or 
other environmental variations. When 200 thirtieth-acre 
plats were planted to a uniform crop of Kherson oats, the 
coefficients of variability for the grain yields of single plats 
and for the mean yields of two, four, and eight plats were 
6.30, 4.59, 2.91, and 2.13 per cent respectively. The extreme 
variation between yields was also reduced from 20.7 bushels 
for single plats to 7.5 bushels for the means of eight plats. 

Reduction of error by averaging adjacent plats (which 
is equivalent to increasing the size of the plat) was far less 
effective than systematic replication. The coefficients of vari- 
ability for single plats and for the mean yields of two, four 
and eight adjacent plats were 6.30, 5.46, 5.28, and 4.78 per 
cent. 

Variation between long, narrow plats was less marked 
than for short, wide plats of the same area. The coefficient 
of variability for tenth-acre oats plats 48 rods by 5.50 feet was 



10 Nebraska Agricultural Exp. Station, Research Bui. 13 

3.84 per cent as compared with 5.18 per cent for plats 16 
rods by 16.5 feet. 

22. Two hundred uniformly planted thirtieth-acre Kher- 
son oats plats were arranged in 50 groups of four adjacent 
plats each, and also in 50 groups of four systematically dis- 
tributed plats. For both methods of grouping, the "prob- 
able error" has been calculated for the mean yield of each 
group of four plats. The results indicate that a small prob- 
able error cannot be regarded as sufficient reason for con- 
fidence in the reliability of data. Because of chance groupings 
of either large or small variations where relatively small 
numbers are used, a mean may be either more or less accurate 
than an application of the probable error would indicate. 

23. In four comparative rate-planting yield tests with 
small grains in alternating single-row plats the probable 
error was less than 2 per cent in all cases, and yet there 
existed an average actual error of 34 per cent in relative 
yields due to plat competition. Similar results are indicated 
for variety tests with small grains. 

24. An application of the probable error to tests made 
in 1916 concerning the relative water requirement for grain 
production of Hogue's Yellow Dent corn and Turkey Red 
winter wheat may result in greatly misplaced confidence. We 
may be confident from one test that Hogue's Yellow Dent 
corn uses considerably less water per pound of grain than 
does Turkey Red wheat, and from another test we may be 
equally confident that the corn uses more than twice as much 
water for grain production as does the wheat. The second 
comparative figures are unreliable because the soil was rela- 
tively overcropped by the corn. 

25. Crop tests are subject to such a multitude of local 
environmental influences that errors in them cannot be 
regarded as occurring according to the formulas or rules of 
chance calculated mathematically from purely mechanical 
observations. The probable error may apply where only 
accidental variations occur but not where systematic varia- 
tions exist. Crop tests are subject to systematic variations. 

26. In view of the precautions necessary to guard against 
the invalidating influences of various sources of experimental 
error, greater and better facilities should be provided experi- 
ment stations for the conduct of crop investigations. 



Experimental Error in Crop Tests 11 

27. In crop breeding experiments improvement in yield 
over the original can only be measured accurately by grow- 
ing each year some of the original unselected seed for com- 
parison. The method of comparing the results of one period 
of years with those of another is unreliable. For example, 
Hogue's Yellow Dent corn which has undergone continuous 
ear-to-row breeding since 1902 yielded 39 per cent less during 
the seven-year period 1907-1913 than during the preceding 
seven years. However, a seven-year comparison with the orig- 
inal seed which has been grown as a check indicates that the 
inherent yielding power of the ear-to-row and the original 
corn are almost identical. 

28. Soil limitation may be a serious source of error in 
pot experiments. The relative total moisture-free yields for 
individual corn plants grown in pots of six sizes in 1914 were, 
in order from the smallest to the largest, 100, 211, 324.1, 
453.6, 643.8, and 747. The corresponding yields of ear corn 
were 100, 632.5, 1082.3, 2417, 2990, and 4046.7. A uniform 
application of 1.75 pound of sheep manure per plant (or 
per pot) increased the yields of total dry matter for the six 
sizes, in order from the smallest to the largest, 176.4, 95.3, 
69.3, 26.1, 12.7, and 7.2 per cent. The corresponding increases 
in yield of ear corn caused by the manure were 722.5, 193.6, 
149.2, 18.9, 14.1, and. 2.9 per cent. 

In 1915 the relative yields of total dry matter from the 
six sizes of pots, progressing from the smallest to the largest, 
were 100, 150, 229.6, 355.6, 586, and 578.7 per cent. The 
corresponding relative yields of ear corn were 100, 276.2, 
819, 1647.5, 2771.3, and 2667. 

Applying manure in amounts proportional to the quan- 
tity of soil contained, in 1915 had far less striking effect 
upon the pot yields for the different sizes than when equal 
quantities were applied in 1914, regardless of the quantity 
of soil contained. 

29. When two, four, or six corn plants were grown in 
pots of the proper size for growing one normal corn plant, 
the individual plant yields of total dry matter were respec- 
tively 50.8, 26.7, and 16.6 per cent as large as for the one-rate, 
while the corresponding yields of ear corn were respectively 
3977, 15.9, and 2.8 per cent as large. 

30. A review of several hundred experiment station 
bulletins dealing with variety, fertilizer, cultural, and pot 



12 Nebraska Agricultural Exp. Station, Research Bui. 13 

tests indicates that the statement of methods employed in 
securing experimental data is often inadequate to acquaint 
the reader with the manner in which the results were ob- 
tained. Such a statement is desirable in order that one may 
judge regarding the reliability of the results and the degree 
of confidence which the data merit. 



STUDIES CONCERNING THE ELIMINATION OF 
EXPERIMENTAL ERROR IN COMPARA- 
TIVE CROP TESTS 



By T. A. KlESSELBACH 



It is apparent that many sources of error have uncon- 
sciously entered into comparative crop yield tests. The very 
important matter of overcoming variation in soil conditions 
as a source of experimental error has been quite extensively 
studied and reported by various investigators during the past 
decade. The means suggested for reducing such error have 
been (1) repetition of plats and (2) correction of yields 
according to check plats planted to a uniform variety or 
treatment at stated intervals. Both methods have proved 
of value and a combination of both may often be used advan- 
tageously. Some danger always exists of error occurring in 
the check plats and that correcting according to them may 
introduce new errors in the yields of crops compared. The 
method should, for this reason, be used with caution. 

Studies in experimental error conducted at this Experi- 
ment Station prior to 1911 have been published by Prof. E. 
G. Montgomery, now of Cornell University, in Bulletin No. 
269, of the Bureau of Plant Industry, U. S. Department of 
Agriculture, and in the Twenty-sixth Annual Report of the 
Nebraska Agricultural Experiment Station. These pub- 
lished results concern primarily the general problems of repe- 
tition and size of nursery small grain plats, and the use of 
check plats. 

The object of the following investigations was to secure 
further information regarding the elimination of error in 
comparative yield tests. Shortage of facilities for carrying 
on this character of work in addition to the regular crop 
investigations of the Experiment Station has in some cases 
necessitated intermittent experiments. The duration of 
some of the tests has for the same reason been shorter than 
would have been desired. 



Acknowledgment is gratefully made to Professor J. A. Ratcliff and Pro- 
fessor C. A. Helm for valuable assistance in field supervision and in keeping- 
records during - much of the time these experiments were in progress. 
Messrs. H. G. Gould, E. R. Bwing, R. E. Holland and H. B. Pier, have also 
rendered efficient assistance at various times. 



14 Nebraska Agricultural Exp. Station, Research Bui. 13 

ERROR DUE TO COMPETITION BETWEEN ADJACENT PLATS 

It is a well known principle in ecology that a keen com- 
petition for soil moisture and nutrients may exist between 
plants which differ in growth habit, when grown in close 
proximity. Competition between adjacent rows of different 
varieties, selections, or rates of planting, had suggested 
itself as a possible source of error in crop tests. An inves- 
tigation was planned in 1912 to determine the relative merits 
of rows and blocks for making comparative yield tests in the 
small grain nursery and in corn experiments. 

The question was : Will two varieties give the same com- 
parative yields when planted in alternating rows as when 
planted in alternating blocks consisting of a number of rows? 
It was reasonable to assume that there would be less plat- 
competition between varieties planted in blocks than when 
planted in single rows. 

It has been a common practice in crop breeding experi- 
ments to compare the selected strains in adjacent one-row 
plats for a number of years. Many other comparative tests 
have also been made in single row plats. 

ILLISTKVTIOX OF PRINCIPLE OP COMPETITION BETWEEN 
ADJACENT ROWS 

On the right-hand side of Fig. 1 is shown a crop of Tur- 
key Red winter wheat planted in the fall of 1912. To the 
south of this was planted Scotch Fife spring wheat in the 
spring of 1913. The first row of spring wheat, spaced ten 
inches from the winter wheat, is seen to have grown only 
about four inches tall with no grain production. The sec- 
ond row of spring wheat made an almost normal growth, 
while the third row was entirely normal. The complete fail- 
ure of the first row of spring wheat may be accounted for 
by the shortage of both moisture and available plant food 
material, due to the more rapid and luxuriant growth of the 
adjacent winter wheat. While this is an extreme example of 
competition between adjacent rows, it illustrates a principle 
commonly applying in crop yield tests. 

COMPETITION BETWEEN ADJACENT HOWS OP SMALL GRAIN 

The plan of the experiment was to plant two crops under 
comparison in alternating one-row plats and alternating five- 
row plats. These were replicated 50 times each year in order 



Experimental Error in Crop Tests 



15 




Fig. 1 — Illustrating principle of competition between adjacent rows. 
Winter wheat on right; spring wheat on left. Due to competition 
with the winter wheat, the first row of spring wheat grew only- 
four inches tall wifh no grain production. The second row was 
nearly normal and the third row entirely normal. 

to eliminate the accidental mechanical and physical errors 
due to variation in soil, exposure, stand, etc. These nursery 
rows were spaced 10 inches apart. The relative yields in 
either the entire five-row block or the three inner rows, as 
indicated, were regarded as the correct relative yields for 
the season. A difference in the relative yields when tested 
in alternating rows, as compared with the relative yields in 
blocks, is chiefly due to, and measures, the competition be- 
tween the crops compared in rows. In part of the tests the 
blocks were harvested as individual rows, which permitted a 
study of the effect of plat competition upon the border rows 
of five-row plats. The straw yields as well as the grain yields 
were also secured in a portion of the tests. 



ROW COMPETITION IN RATE-OF-PLANTING TESTS WITH 
WHEAT AND OATS 

During the jrears 1913 and 1914, both oats and winter 
wheat were grown at two distinct rates of planting in both 



16 Nebraska Agricultural Exp. Station, Research Bui. 13 

alternating single-row plats and alternating five-row nur- 
sery plats, 16 feet in length. 

Wheat — Table 1 shows the results with the wheat rate- 
of-planting tests. 

When grown in single rows in 1913, the thin rate yielded 
68 per cent as much as the thick rate, while in five-row 
blocks the thin rate yielded 90 per cent as much as the thick 
rate. Competition in rows with a thicker rate of planting 
caused the thin rate to yield relatively 24.4 per cent too low. 
(This percentage effect of competition is determined by 
dividing the difference between 68 per cent and 90 per cent, 
or 22, by 90.) 

In 1914 the thin rate in rows yielded 35 per cent as much 
as the thick rate, while in the center three rows of five-row 
plats it yielded 81 per cent as much as the thick rate. Due 
to competition, the thin rate yielded 56.8 per cent too low. If 
the two outside rows are averaged into the block yield, the 




Fig. 2 — Method of planting nursery small grain plats with a special 
nursery drill. The drill can be rapidly adjusted to plant each row 
at a given rate, independently of the other rows 



Experimental Error in Crop Tests 



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ratio of thick to thin is 100:72 as compared with 100:81 for 
the center three rows, while the ratio of thick to thin for the 
two outside rows only was 100:60. From these data and 
other similar data it may be concluded that the outside rows 
of nursery test plats should be discarded. 

The straw yields for the 1914 rate-of-planting tests with 
wheat substantiate the same principles of competition as were 
brought out in the relative grain yields. In alternating rows, 
the ratio of thick to thin straw yield was 100:39. For the 
center three rows of five-row blocks, the ratio was 100:82. 
The ratio was 100:74 where all five rows were averaged, 
while it was 100 :63 for the two outside rows. 

Oats — The relative yields of two rates of planting oats 
in alternating rows as compared with alternating five-row 
plats are shown in Table 2. In 1913 the thin rate in rows 
yielded 64 per cent as much as the thick rate, while in five- 
row blocks the thin rate yielded 80 per cent as much as the 
thick rate. Competition in rows with a thicker rate of plant- 
ing caused the thin rate to yield relatively 20 per cent too low. 

In 1914 the thin rate in alternating rows yielded 67 per 
cent as much as the thick rate, while when compared in the 
three inner rows of five-row plats the thin rate yielded 2 per 
cent more than the thick rate. Competition in rows with the 
thicker rate caused the thin rate of planting to yield rela- 
tively 34.3 per cent too low. If the yields of the entire five- 
row blocks are taken, the ratio of thick to thin is found to 
have been 100:98 as compared with 100:102 for the three 
inside rows, while the ratios of thick to thin for the two out- 
side rows was 100:96. 

Similar results were obtained from the straw yields in 
1914. In alternating single rows the ratio of thick to thin 
straw yields was 100:69. For the center three rows of five- 
row blocks the ratio was 100:102. Where all five rows were 
averaged the ratio was 100:101, while for the two outside 
rows it was 100 :99. 

RELATIVE STOOLING OF TWO RATES OF PLANTING WHEN COM- 
PARED IN ALTERNATING ROWS AND ALTERNATING BLOCKS 

In 1914, counts were made to determine the effect of com- 
petition between alternating rows of two rates of planting 
wheat and oats upon the relative stooling in the two rates. 
The counts were made for the plats reported in Tables 1 
and 2. The results are given in Table 3. 



Experimental Error in Crop Tests 



19 



Table 3 — Relative stooling of two rates of planting with Tur- 
key Red Wheat and Kherson oats when compared in 
alternating one-row plats and alternating five-row plats 
(19U). 



Character of plats and rate of planting 


No. 

plants 

in 10 feet 

of row 


No. stools 

in 10 feet 

of row 


No. stools 
per plant 


WHEAT 1914 
One-row plats 

Thick rate 


140 

52.5 
100:37 

150 

50.5 
100:34 

195.5 
100.5 
100:51 

195 
100 
100:51 


620 

281 

100:45 

560 
364 
100:65 

392.5 
271.0 
100:69 

380 
320 
100:84 


4.4 


Thin rate 


5.4 


Ratio thick to thin 


100:123 


Five-row plats (middle 3 rows) 

Thick rate 


3.7 


Thin rate 


7.2 


Ratio thick to thin 


100:195 


OATS 1914 
One-row plats 

Thick rate 


2.0 


Thin rate 


2.7 


Ratio thick to thin 


100:135 


Five-row plats (middle 3 rows) 

Thick rate 


1.9 


Thin rate T 


3.2 


Ratio thick to thin 


100:168 



In the alternating rows of wheat, the actual number of 
plants per row were in the ratio of 100:37, while in the 
three inside rows of the five row plats the ratio was 100:34. 
The number of culms per plant in the alternating thick and 
thin rows were in the ratio of 100:123, while in the center 
three rows of the five row plats the ratio was 100:195. 

In the case of the oats, the actual number of plants per 
row were in the ratio of 100:51, both for the alternating 
rows and for the three inside rows of the five-row blocks. 
The number of culms per plant in the alternating thick and 
'Jiin rows were in the ratio of 100:135, while for the center 
three rows of the five-row plats the ratio was 100:168. 

ROW COMPETITION BETWEEN VARIETIES OF WHEAT AND OATS 

Wheat — During the years 1913 and 1914, Big Frame 
winter wheat was compared with Turkey Red winter wheat 
in both alternating single-row plats and alternating five-row 



20 Nebraska Agricultural Exp. Station, Research Bui. 13 



'. 


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Fig. 3 — Competition between two types of wheat in adjacent rows. 
The single-row method of testing is unreliable 




Fig. 4 — The "block" method of comparing varieties or selections for 
yield in the nursery. The two outside rows of each block should be 
discarded in order to avoid error from competition between adjacent 
plats. Part of the plats have been harvested 



Experimental Error in Crop Tests 21 

plats. A similar comparison was also made between Turkey 
Red and Nebraska No. 28 winter wheat. 

Turkey Red is the standard bearded hard winter variety 
for normal Nebraska conditions, while Big Frame is one of 
the best beardless varieties of rather similar growth habits. 
The Nebraska No. 28 is an early wheat ripening about ten 
days before Turkey Red, and is normally six inches shorter. 
The relative growths of these varieties differ somewhat in 
different years according to their response to varying cli- 
matic conditions. This will account for one variety outyield- 
ing in one season, and another variety in a different season. 
For example, in 1913 the Nebraska No. 28 wheat grew fully 
as tall as Turkey Red, because it had attained its maximum 
height before dry weather set in, which somewhat stunted the 
more slowly developing Turkey Red wheat. The season of 
1914 was more favorable for the Turkey Red wheat, which 
produced a normal, relatively greater vegetative growth. 

Table 4 gives the two years' results with Turkey Red 
and Big Frame wheat. When grown in alternating single 
rows in 1913, the Big Frame yielded 7 per cent more grain 
than the Turkey Red wheat, while in alternating five-row 
plats, the Big Frame yielded 3 per cent less than the Tur- 
key Red. Due to competition, the Big Frame yielded rela- 
tively 10.3 per cent too high in single-row plats. 

In 1914, the Big Frame yielded 85 per cent as much 
grain as Turkey Red when compared in alternating one-row 
plats, while it yielded 97 per cent as much in five-row plats. 
Competition in rows with Turkey Red caused the Big Frame 
to yield relatively 12.4 per cent too low. 

The straw yields for 1914 give results similar to those 
for grain. In alternating rows the ratio of Turkey Red to 
Big Frame straw yields was 100:90. In five-row plats this 
ratio was 100:97. 

Table 5 gives the relative yields of Turkey Red and Ne- 
braska No. 28 wheat during 1913 and 1914. The ratio of 
Turkey Red to Nebraska No. 28 grain yield was 100:107 in 
1913, both when grown in alternating single-row plats and 
alternating five-row plats. The growth of the two varieties 
this year was so similar that competition appears to have 
been a negligible factor. 

In 1914 the Nebraska No. 28 yielded 63 per cent as much 
as the Turkey Red when compared in alternating single-row 
plats, while it yielded 85 per cent as much in alternating 



22 Nebraska Agricultural Exp. Station, Research Bui. 13 



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Experimental Error in Crop Tests 23 

five-row plats. In rows competition caused the Nebraska No. 
28 to yield relatively 25.9 per cent too low. In this test the 
Nebraska No. 28 five-row plats were harvested as separate 
rows. The center three rows, free from competition with the 
ranker growing Turkey Red variety, yielded 21.0 per cent 
more per row than did the two outside rows. The three 
inside rows also yielded 7.7 per cent more per row than did 
the entire five-row plat. 

The straw yields for 1914 indicate similar effect of com- 
petition. Compared in alternating single-row plats, the ratio 
of Turkey Red to Nebraska No. 28 straw yields was 100:53, 
while in five-row plats this ratio was 100 :80. The center 
three rows yielded 19.1 per cent more straw per row than 
did the two outer rows, which were obliged to compete with 
Turkey Red. The center three rows also yielded relatively 
6.9 per cent more straw per row than did the entire five- 
row plat with the two outside rows included. 

Oats — Both Burt and Swedish Select oats varieties were 
compared during 1913 and 1914 with Kherson oats in alter- 
nating single-row and alternating five-row plats. 

Kherson oats is the standard early variety grown at the 
Nebraska Experiment Station. Burt oats is rather similar 
in growth habit to the Kherson, ripening at about the same 
time. The Swedish Select is a somewhat taller variety, ripen- 
ing about ten days later. 

Table 6 gives the two years' results with Kherson and Burt 
oats. In 1913 the Burt outyielded the Kherson 30 per cent 
when planted in alternating single rows and 12 per cent in 
alternating five-row plats. Due to competition the Burt 
yielded relatively 16 per cent too high in single-row plats. 

In 1914 the Burt yielded 39 per cent more than the Kher- 
son in alternating single row plats, while it yielded 1 per 
cent more in the three center rows of alternating five-row 
plats. Competition in rows with Kherson oats caused the 
Burt to yield relatively 37.6 per cent too high. If the yields 
of the entire five-row plats are taken, the ratio of Kherson 
to Burt oats is 100:109 as compared with 100:101 for the 
three inside rows, and 100:120 for the two outside rows. 

The straw yields which were obtained for 1914 gave 
very similar results. In alternating single rows the ratio 
of Kherson to Burt straw yields was 100:139. For the three 
inside rows of alternating five-row plats the ratio was 
100:109. For the entire five-row plats the ratio was 100:117. 
For the two outside rows it was 100 :129. 



24 Nebraska Agricultural Exp. Station, Research Bui. 13 










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Experimental Error in Crop Tests 



25 



Table 7 summarizes the two years' data with Kherson 
and Swedish Select oats. In 1913 the Swedish Select yielded 
18 per cent less than the Kherson when grown in alternating 
single-row plats, and 23 per cent less in alternating five-row 
plats. In alternating single rows the Swedish Select yielded 
relatively 7 per cent too high. 

In 1914 the Swedish Select yielded 89 per cent as much 
grain as Kherson in alternating single-row plats and 93 per 
cent as much in five-row plats. The Swedish Select straw 
yielded 13 per cent more in alternating rows and 17 per cent 
more in five-row plats. 

EVIDENCE OF PLAT COMPETITION IN A WHEAT-BREEDING NURSERY 

During the four years, 1910, 1911, 1912 and 1914, 80 
strains of Turkey Red wheat were tested at the ordinary 
field rate of seeding in identically the same order each year, 
in single 16-foot rows ten inches apart. The entire series 
has been replicated ten times each year. It is probable that 
many of the yields have been subject to the effect of row com- 
petition. 

Table 8 contains a concrete example of competition be- 
tween strains in such a wheat-breeding nursery. In the four- 
year row test of 80 strains, strain No. 75 ranked 80, while 
strains No. 74 and No. 76 on either side ranked 1 and 5. 
Strain No. 75 is a slightly shorter and thinner stooling type. 
To determine whether the relative rankings of these strains 
might have been influenced by competition, they were com- 
pared in both rows and blocks for two years, 1915-1916. 



Table 8— Relative yields of three Turkey Red wheat strains 
when compared in five-row nursery plats and in single- 
rovj plats. Tivo-year average (1915-1916) 





Strain number 




Relative yields 






Blocks 


Rows 






GRAIN 






74 
75 
76 




106 
100 
108 


126 
100 
123 






STRAW 






74 
75 
76 






110 
100 
102 


113 
100 
109 



26 Nebraska Agricultural Exp. Station, Research Bid. 13 

Strain No. 74 was favored 20 per cent and strain No. 76, 15 
per cent in yield by being compared (with an adjacent less 
vigorous type) in rows rather than in blocks. Fig. 5 is a 
photograph of these strains. 




Strain No.: 



Ck. 



74 



Fig. 5 — Single-row nursery test plats of Turkey Red Winter wheat. 
Strain No. 75, in center, is seen to have a lower stooling capacity 
and is given and unfair test when growing between two high-stool- 
ing strains. The two adjacent strains in turn have an unfair advan- 
tage 



Experimental Error in Crop Tests 



27 



These 80 strains are now all being grown in five-row 
plats, replicated ten times, for the purpose of establishing 
the correct relative yields, free from competition as a source 
of experimental error. Single-row plats are now regarded 
as unreliable and misleading, because a strain is certain to 
be unduly favored when grown beside a strain lower in com- 
petitive qualities due to such factors as low stooling, slow 
growing, or partial winterkilling. It is important to have 
any crop being tested surrounded by a crop of its own kind. 

COMPETITION BETWEEN INDIVIDUAL PLANTS 

Altho the yields of small grain are never compared by 
planting alternating seeds of two varieties or two grades of 
seed in the same row, yet such a comparison may be of inter- 
est to throw further light upon the principle of competition. 

Table 9 — Relative yields, at the normal field rate of planting, 
of equal numbers of large and small wheat seeds when 
grown alone in blocks and, when grown in competition by 
alternation in the same row* 



Method of comparing large and small 
seeds 



Ratio of yield of small seeds to 
large seeds 



Grain 



Straw 



Total 



WINTER WHEAT, 1914 

Grades alone in blocks I 90:100 I 94:100 

Grades competing | 61:100 72:100 

WINTER WHEAT, 1915 

Grades alone in blocks I 99:100 I 98:100 

Grades competing | 83:100 | 78:100 

SPRING WHEAT, 1914 

Grades alone in blocks I 88:100 I 93:100 

Grades competing I 78:100 | 78:100 

SPRING WHEAT, 1915 

Grades alone in blocks I 80:100 I 93:100 

Grades competing | 82:100 | 73:100 

AVERAGE FOR WINTER AND SPRING WHEAT, 1914-1915 

Grades alone in blocks ] 89:100 I 94:100 

Grades competing | 76:100 \ 75:100 

*Compiled from data in Nebraska Research Bulletin No. 11, 1917 



94:100 
71:100 



98:100 
79:100 



92:100 
78:100 



90:100 
75:100 



93:100 
76:100 



28 Nebraska Agricultural Exp. Station, Research Bui. 13 



During 1914 and 1915 large and small wheat seeds were 
planted alternatingly in the row at the normal field rate of 
planting. Two varieties were used and reciprocated so that 
the results in Table 9 represent the mean of two varieties for 
each grade. This reciprocation eliminates largely the varietal 
effects in the summary. It was necessary to use two distinct 
varieties (a bearded and a beardless) so that the plants from 
each grade might be separated at harvest. The same grades 
were also compared separately in nursery blocks to establish 
the relative yields when free from competition. 

As an average for two varieties each of winter and spring 
wheat for two years, the small seed in competition yielded 
relatively 15 per cent too little grain, 20 per cent too little 
straw, and made 18 per cent too small total yield. 

Table 10 — Relative yields at the normal field rate of planting, 
of two varieties when grown alone in blocks, and when 
grown in competition by alternation in the same row* 



Method of comparing varieties 



Relative yields 



Grain 



Straw 



Total 



Ratio Big Frame 

to Turkey Red \ Competition 



WINTER WHEAT, 1914 
/Alone... 90:100 



Ratio Scotch Fife 

to Marquis \ Competition 



55:100 

SPRING WHEAT, 1914 
/Alone | 75:100 



Ratio Big Frame / Alone 

to Turkey Red \ Competition 



61:100 
WINTER WHEAT, 1915 

82:100 
120:100 



SPRING WHEAT, 1915 



Ratio Scotch Fife / Alone 

to Marquis 1 Competition . 



95:100 
99:100 



88:100 
70:100 



93-100 
90:100 



105:100 
128:100 



114:100 
125:100 



89:100 
67:100 



90:100 
86:100 



99:100 
125:100 



109:100 
119:100 



*Compiled from data in Nebraska Research Bulletin No. 11, 1917. 

The results for different years should not be averaged in this variety 
test, since varieties do not have the same relative competitive qualities 
in different years. We are interested here in what may happen any one 
year and not in an average of years. 

In similar manner, competition between two varieties 
planted within the same row was determined. Plants from 
each variety could be separated at harvest by the presence 
or absence of beards. The relative yields were also obtained 
in nursery blocks free from competition by harvesting the 



Experimental Error in Crop Tests 



29 



three inside rows of five-row blocks. The results in Table 
10 indicate marked competition between varieties. Variety 
competition amounted to 61 per cent and 46 per cent for win- 
ter wheat yields in 1914 and 1915 respectively. For spring 
wheat this competition equaled 19 per cent and 4 per cent 
in 1914 and 1915 respectively. 

COMPETITION BETWEEN CORN TEST PLATS AS A SOURCE OP 
EXPERIMENTAL, ERROR 

In corn variety tests, corn breeding experiments, and 
various other corn yield tests the crops under comparison 
have customarily been planted in adjacent plats containing 
one, two, three, or four rows. The single-row plat is used 
almost universally in corn breeding experiments. In several 
instances where only three or four kinds of corn were to be 
compared, these have all been planted in the same hill, giving 
each kind of corn a definite position in the hill. This intra- 
hill method has been employed by Hartley, Brown, Kyle, and 
Zook (1912) and by Collins (1914).* 




Fig. (i — Planting experimental corn plats where accuracy is required. 
Hand planters are found tar superior to planting with a hoe. A 
stated number of kernels are placed in the planter for each drop 



*The year in parentheses following- an author's name in the text serves 
to associate the reference with a particular publication in the Bibliography 
(pp. 91-94), where the complete title is given. 



30 Nebraska Agricultural Exp. Station, Research Bui. 13 




Fig. 7 — A hill of checked corn with the three plants spaced in the hill in 
order that the plants may be readily counted without suckers being 
mistaken for separate plants 

In 1912 the Nebraska Experiment Station commenced a 
series of experiments to determine the reliability of the vari- 
ous kinds of corn test plats. The investigations were extended 
in 1913 but the corn was not harvested because of an almost 
total crop failure due to deficient rainfall. Good results were 
secured in 1914, 1915, and 1916. 

For planting, the land was marked off into hills three feet, 
eight inches apart and the corn planted at double the desired 
rate by means of hand planters. (Fig. 6.) When about four 
inches high the plants were thinned to the desired rate, thus 
producing an almost perfect stand. The plants were spaced 
within the hills so that the original plants could be easily 
distinguished from suckers. For the comparative yield tests, 



Experimental Error in Crop Tests 



31 



50 hills with the desired number of plants and surrounded 
by a normal stand were harvested from each row. This was 
accomplished by planting 72 hills in each row, which per- 
mitted the elimination of any hills having less than the full 
stand. Thus all yields were comparable so far as number 
of plants was concerned. The plats have been replicated 
eight or more times each year, as indicated in the tables, 
in order to eliminate soil variations. 

ROW COMPETITION IN RATB-OF-PLANTING TESTS WITH CORN 

Tables 11, 12, and 13 contain three years' results with 
planting Nebraska White Prize corn at the rate of two and 
four plants per hill in alternating single-row and three-row 




Fig. 3 — A hill of checked corn planted by the ordinary method without 
spacing the plants in the hill. It contains two plants, altho the 
number cannot be readily nor accurately determined as with the 
space-planted hill 



32 Nebraska Agricultural Exp. Station, Research Bui. 13 

Table 11 — Relative yields of two rates of planting with Ne- 
braska White Prize corn when compared in alternating 
one-row plats and in alternating three-row plats (191U) 













Yield per acre 


No. of 

rows in 

plat 


No. of 
plants 
per hill 


No. of 
replica- 
• tions 


No. of 
suckers 
per 100 

plants 


No. of 

ears 

per 100 

plants 


Average 
One-row plat or of two 
center row ' outside 
rows 


1 
1 


4 
2 


15 
15 


7.4 
26.6 


67.0 
93.0 


Bushels 
43.8 
35.6 


Per cent 
100.0 
82.0 


Bushels 


3 
3 


4 
2 


9 
9 


7.1 
32.3 


66.0 
96.0 


38.4 
44.3 


100.0 
116.0 


39.8 
42.4 



plats. The rows were harvested separately in the three-row 
plats. 

In 1914 the two-rate yielded 18 per cent less than the 
four-rate when compared in alternating single-row plats. In 
the center rows of alternating three-row plats, the two-rate 
yielded 16 per cent more than the four-rate. Due to compe- 
tition with a thicker stand, the two-rate yielded relatively 
29.3 per cent too low in alternating single-row plats. In the 
two outer rows of the three-row plats, the ratio of the four- 
rate to the two-rate was 100:106.5 as compared with 100:116 
for the center rows. 



Table 12 — Relative yields of two rates of planting with Ne- 
braska White Prize corn when compared in alternating 
one-row plats and alternating three-row plats (1915) 













Yield per acre 


No. of 


No. of 


No. of 


No. of 


No. of 




rows in 


plants 


replica- 


suckers 


ears 




Average 


plat 


per hill 


tions 


per 100 


per 100 


One-row plat or 


of two 








plants 


plants 


center row 


outside 














rows 












Bushels 


Per cent 


Bushels 


1 


4 


8 


8.5 


95 


101.7 


100.0 




1 


2 


8 


21.8 


110 


64.2 


63.1 




3 


4 


8 


11.9 


93 


90.0 


100.0 


91.2 


3 


2 


8 


29.7 


112 


62.0 


70.0 


63.0 



Experimental Error in Crop Tests 



33 



In 1915 (Table 12), the two-rate yielded 36.9 per cent less 
than the four-rate when compared in alternating single-row 
plats. In the center rows of alternating three-row plats the 
two-rate yielded 30 per cent less than the four-rate. Due to 
competition, the two-rate yielded relatively 9.9 per cent too 
low in single-row plats. In the two outer rows the ratio of 
the four-rate to the two-rate was 100:69 as compared with 
100 :70 for the center rows. Competition was far less marked 
in 1915 than in 1914 because of much more favorable moist- 
ure conditions. 

In 1916 (Table 13), the two-rate yielded 21.3 per cent less 
than the four-rate when compared in alternating single-row 
plats. In the center rows of alternating three-row plats the 
two-rate yielded 6.2 per cent less than the four-rate. As the 
result of competition, the two-rate yielded relatively 16.1 per 
cent too low in single row plats. In the two outer rows the 
ratio of the four-rate to the two-rate was 100:85.9 as com- 
pared with 100:93.8 for the center rows. 

Table 13 — Relative yields of two rates of planting with Ne- 
braska White Prize corn when compared in alternating 
one-row plats and alternating three-row plats (1916) 



No of 


No. of 
plants 
per hill 


No. of 
replica- 
tions 


No. of 
suckers 
per 100 

plants 


No. of 

ears 
per 100 
plants 


Yield per acre 


rows in 
plat 


One-row plat or 
center row 


Average 

of two 

outside 

rows 


1 
1 


4 

2 


8 
8 


24.8 
62.5 


82 
107.1 


Bushels 
52.7 
41.5 


Per cent 
100 
78.7 


Bushels 


3 
3 


4 
2 


8 

8 


23.0 
60.0 


79.9 
115.6 


51.8 
48.6 


100 
93.8 


53.4 
45.9 



INTRA-HILL AND ROW COMPETITION IN CORN VARIETY YIELD TESTS 



During the years 1912 and 1914, Pride of the North corn 
was compared with Hogue's Yellow Dent corn in (1) alter- 
nating single rows, (2) alternating three-row plats, and (3) 
in the same hill. A similar comparison was also made be- 
tween University No. 3 corn and Hogue's Yellow Dent in 
1914. The relative yields of the above three varieties were 
also determined by planting all in the same hill. 



34 Nebraska Agricultural Exp. Station, Research Bui. 13 

The relative growth habits of these three varieties dur- 
ing 1914 is shown in Table 14. Hogue's Yellow Dent is a 
large variety of corn requiring the entire season to mature. 
Pride of the North is a small, early-maturing variety. Uni- 
versity No. 3 is normally somewhat earlier and smaller than 
Hogue's Yellow Dent. 

Table 14 — Relative growth characters of three corn varieties 
used in 191 U (Table 16) to determine the amount of error 
from variety competition when tested by the single-row 
and intra-hill methods (191 U) 



Variety 



Hogue's Yellow Dent 

University No. 3 

Pride of the North . . 



Length of 
growing 
season 



Days 

119 

107 

92 



Height of 
stalk 



Inches 
96 
92 
70 



Leaf-area 
per plant 



Sq. In. 
997 
940 
408 




Fig. 9 — Alternating single-row plats of Hogue's Yellow Dent and Pride 
of the North corn, 1914. The row method of testing corn types 
which differ in growth habit is unreliable because of competition 
between the plats 



Experimental Error in Crop Tests 



35 




Fig. 10 — Alternating three-row plats of Hogue's Yellow Dent and Pride 
of the North corn, 1914. Pride of the North on the right. Compe- 
tition between test plats may be avoided and correct relative yields 
obtained by discarding the outside rows of three-row plats 



In 1912 Hogue's Yellow Dent and Pride of the North 
corn were grown in alternating single rows and in alternating 
three-row plats at the rate of three plants per hill in each case. 
These were also compared for yield by growing one plant of 
each variety in the same hill. For this reason the variety 
yields per acre in the hill method are on a different basis than 
in case of the rows and blocks, but nevertheless they are com- 
parable. The, three-row plat tests were replicated 10 times, 
the single row plats 20 times, and the hills 1,000 times. The 
results are contained in Table 15. 

In alternating three-row plats, Pride of the North yielded 
85 per cent as much as Hogue's Yellow Dent, while in alter- 
nating single-row plats it yielded 66 per cent as much as the 
Hogue's Yellow Dent. Within the same hill, Pride of the 
North yielded 47 per cent as much as Hogue's Yellow Dent. 
Due to competition Pride of the North yielded relatively 44.7 
per cent too low in the same hill, and 22.4 per cent too low 
in the alternating rows. 



36 Nebraska Agricultural Exp. Station, Research Bui. 13 

In 1914 Hogue's Yellow Dent corn was compared with 
University No. 3 corn in addition to a comparison with Pride 
of the North as made in 1912. All three varieties were also 
compared in the same hill. Plats were replicated the same 
as in 1912. The results are contained in Table 16. 

In the center row of alternating three-row plats, Pride 
of the North yielded 53 per cent as much as Hogue's Yellow 
Dent, while in alternating single row plats it yielded 38 per 
cent as much as Hogue's Yellow Dent. Within the same 




Fig. 11 — Relative growth of Hogue's Yellow Dent, University No. 3, 
and Pride of the North corn varieties when grown in the center 
row of three-row plats (1914) 



Experimental Error in -Crop Tests 



37 



hill, Pride of the North yielded 26 per cent as much as 
Hogue's Yellow Dent. Due to competition with Hogue's Yel- 
low Dent in the same hill, Pride of the North yielded rela- 
tively 51 per cent too low, while in alternating single-row 
plats it yielded relatively 28.3 per cent too low. 

Comparing the yields of Hogue's Yellow Dent and Uni- 
versity No. 3 in the center rows of alternating three-row 
plats we have a ratio of 100:98, while in alternating single- 
row plats this ratio was 100:90. In the same hill the ratio 




Fig. 12 — Relative growth of Hogue's Yellow Dent, University No. 3, 
and Pride of the North corn varieties when grown in the same hill 
(1.914) 



38 Nebraska Agricultural Exp. Station, Research Bui. 13 







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Experimental Error in Crop Tests 



39 



was 100:99. Due to competition, the University No. 3 
yielded relatively 8.0 per cent too low, in single rows and 
within the same hill it yielded 1 per cent too high. The ap- 
parent lack of competition within the hill in this case may 
have been due to there being only two plants of rather similar 
type in a hill. 

When all three varieties were compared in the same hill 
the relative yields for the Hogue's Yellow Dent, University 
No. 3, and Pride of the North were respectively 100, 96, and 
28, as compared with 100, 98, and 53 in the center rows of 
three-row plats, and 100, 90, and 38 in single-row plats. 

In the three-row plats (Table 16), the yields indicate 
that competition affects the outer rows to such an extent that 
they should be discarded in all yield tests of corns which dif- 
fer in growth habit. Single-row plats are unreliable for a 
comparative test of corn differing in growth habit or rate of 
planting. Two-row plats would probably be subject to one- 
half of the competition of single-row plats. 

In 1913 (Table 17), inbred and first generation hybrid 
Hogue's Yellow Dent corn were similarly compared in (1) 
alternating single rows, (2) alternating three-row plats, and 
(3) in the same hill. The inbred corn had been self-fertilized 



Table 17 — Relative yields of inbred Hogue's Yellow Dent corn 
and first generation hybrid seed of inbred strains when 
compared in three-row plats, single-row plats, and when 
planted in the sam,e hill (1916) 





Plants 
per hill 


No. of 
replica- 
tions 


Yield per acre 


Manner of planting 


Actual Relative 


Cross- 
bred 


1 


Inbred 


Crossbred and inbred strains 
of H. Y. D. corn alternat- 
ing in 3-row plats 

Crossbred and inbred strains 
of H. Y. D. corn alternat- 
ing in single rows 

Crossbred and inbred strains 
of H. Y. D. corn planted 
in the same hill* 


4 
4 
4 


9 

6 

300 


Bus- 
hels 

76.2 

90.5 

54.0 


Bus- 
hels 

28.1 

28.0 

11.2 


Per 

cent 

100 
100 
100 


Per 

cent 

36.9 
31.1 
20.7 



*Where two plants each of two types were grown in the same hill, the 
actual yield for each type is given, based on the rate of two plants per hill. 



40 Nebraska Agricultural Exp. Station, Research Bui. 13 



Table 18 — Summary of relative grain yields when different 
rates of planting are tested in single-row plats and also in 
blocks containing several rows 



Crop tested at two rates of planting 



Turkey Red winter wheat. 
Turkey Red winter wheat . 

Kherson Oats 

Kherson Oats 

Nebraska White Prize corn 
Nebraska White Prize corn 
Nebraska White Prize corn 



Ratio thick to thin 



Year of 

test 


Alternat- 


Alternat- 




ing rows 


ing 
blocks 


1913 


100:68 


100:90 


1914 


100:35 


100:81 


1913 


100:64 


100:80 


1914 


100:67 


100-102 


1914 


100:82 


100:116 


1915 


100:63 


100:70 


1916 


100:78 


100:93 



Table 19 — Summa7*y of relative grain yields when different 
varieties are tested in single-row plats and also in blocks 
containing several rows 



Varieties compared in 

alternating rows and in 

alternating blocks 


Year of 
test 


Ratio of variety No. 1 to 
variety No. 2 in 


Alternat- 
ing rows 


Alternat- 
ing 
blocks 


Compet- 
ing in 
same hill 
(Corn) 


Turkey Red (1) and Big Frame 
(2) winter wheat 


1913 

1914 

1913 

1914 
1913 
1914 

1913 

1914 
1912 
1914 
1914 
1916 


100:107 

100:85 

100:107 

100:63 

100:130 

100:139 

100:82 

100:89 

100:66 

100:38 

100:90 

100:31 


100:97 

100:97 

100:107 

100:85 

100:112 

100:101 

100:77 

100:93 

100:85 

100:53 

100:98 

100:37 




Turkey Red (1 ) and Big Frame 

(2) winter wheat 

Turkey Red (1) and Nebraska 
No. 28 (2) winter wheat. . . 
Turkey Red (1) and Nebraska 
No. 28 (2) winter wheat. . . . 
Kherson (1) and Burt (2) oats 
Kherson (1) and Burt (2) oats 
Kherson (1) and Swedish Se- 
lect (2) oats 




Kherson (1) and Swedish Se- 
lect (2) oats 




Hogue's (1) and Pride of the 
North (2) corn 


100:47 


Hogue's (1) and Pride of the 
North (2) corn 


100:26 


Hogue's (1) and University 
No. 3 (2) corn 


100:99 


Fi* Hogue's (1) and inbred 
Hogue's (2) corn . . . 


100:21 











'First generation hybrid of inbred strains. 



Experimental Error in Crop Tests 41 

for five years and was greatly reduced in size and vigor. 
The results indicate the error which might be expected if 
two inbred parents were to be compared with their hybrid 
and the original check seed. In alternating three-row plats, 
the inbred corn yielded 38.9 per cent as much as the hybrid 
seed, while in the alternating single-row plats it yielded 31.1 
per cent as much. When compared in the same hill, the 
inbred seed yielded 20.7 psr cent as much as the hybrid seed. 
Because of competition with the larger plants in the same 
hill, the inbred corn yielded relatively 44 per cent too low. 
while in alternating single rows, it yielded relatively 16 per 
cent too low. 

SUMMARY OF PLAT COMPETITION STUDIES 

The effects of single row plat competition upon compara- 
tive grain yields, are summarized for wheat, oats, and corn, 
in Tables 18 and 19. These data are taken from Tables 1 
to 7 and 11 to 17. The ratios given for the comparative 
yields in blocks are for the middle row or middle three rows 
of either three-row plats or five-row plats, except in 1913, 
when the block-rows were not harvested separately. 

VARIATION OF STAND AS A SOURCE OF ERROR IN YIELD 
TESTS WITH CORN 

In order to secure information regarding the effect of 
variation in stand upon the accuracy of comparative corn 
tests, 2,000 hills of corn were planted in 1914 and 8,500 hills 
in 1917, in which were methodically distributed two, one and 
no-plant hills among hills with a full stand of three plants. 
Each hill was harvested separately. The results are contained 
in Tables 20 and 21. 

In 1914 (Table 20), when surrounded by hills having a 
full stand of three plants, the respective relative grain yields 
of three-plant, two-plant and one-plant hills were 100, 82, 
and 74. In 1917 the corresponding relative yields were 100, 
83, and 50. 

In 1914 (Table 21), when three-plant corn hills, other- 
wise surrounded by a full stand of three plants per hill, were 
adjacent to (1) one hill with two plants, (2) one hill with 
one plant, (3) one blank hill, (4) two blank hills, the respec- 
tive grain yields per hill were 3 per cent, 5 per cent, 13 per 
cent and 43 per cent greater than when surrounded entirely 
by three-plant hills. 



42 Nebraska Agricultural Exp. Station, Research Bui. 13 



In 1917 corresponding hills with missing plants increased 
the grain yields of three-plant hills respectively 2 per cent, 
9 per cent, 15 per cent and 25 per cent over the yield of 
three-plant hills entirely surrounded by three-plant hills. 

The data indicate that irregularity of stand in corn yield 
tests may cause inaccurate yields and should be avoided. 

Error due to variation in stand of corn may be largely 
overcome by planting the corn thick and thinning to a uni- 
form stand soon after coming up. If grown in hills, the 
seed may be space-planted in the hill so that the actual 
number of plants may be readily counted at harvest with- 
out suckers being mistaken for separate plants. It is desir- 
able, just before husking, to count out a given number of 
hills having a full stand and surrounded by a normal stand, 
upon which to base the yield per acre. This may be facili- 
tated by planting an additional number of hills to permit dis- 
carding. Space-planting in the hill for experimental yield 
tests may be accomplished by first marking off the field cross- 
wise with a sled marker and then making three separate 
spaced plantings in each intersection by means of a hand 
corn planter adapted for the purpose. Where three plants 
are grown per hill, the marker runners should be double 
so that all three plantings may be made in a runner mark, 
thus insuring uniform planting conditions for all three plants. 
There are exceptional kinds of corn experiments in which 
planting thick and thinning to insure a perfect stand would 
conflict with the object of the investigation. 

Table 20 — Relative yields of one, two, and three-plant corn 
hills when surrounded, uniformly by three-plant hills 
(1914 and 1917) 



Number of plants in 

hills surrounded by 

uniform three-plant 

hills 


Number 
of hills • 
averaged 


Number 

of tillers 

per 100 

plants 


Number 
of ears 
per 100 
plants 


Average grain 
yield per hill 


Actual 


Relative 


Hills with three plants?.. 
Hills with two plants . . . 
Hills with one plant. . . . 

Hills with three plants. 


YI 

310 
70 
16 
YE 

288 


]AR 1914 
8 
38 
112 
]AR 1917 


83 

96 

168 

95 
102 
114 


Grams 

466 
380 
344 

509 

422 
252 


Per cent 

100 

82 

74 

100 


Hills with two plants. . . 


50 
64 




83 


Hills with one plant. . . . 




50 



Experimental Error in Crop Tests 



43 



Table 21 — Relative yields of three-plant corn hills adjacent 
to hills with missing plants (1911+ and 1917) 



Three-plant hills sur- 
rounded by three- 
plant hills except as 
indicated below 



Number 

of hills 

averaged 



Surrounded by hills with 

three plants 

Adjacent to one hill with 

two plants 

Adjacent to one hill with 

one plant 

Adjacent to one blank 

hill 

Adjacent to two blank 

hills 

Surrounded by hills with 

three plants 

Adjacent to one hill with 

two plants 

Adjacent to one hill with 

one plant 

Adjacent to one blank 

hill 

Adjacent to two blank 

hills 



Number 

of plants 

per hill 



Number 
of ears 
per 100 
plants 



YEAR 1914 



YEAR 1917 



Average grain 

yield of three-plant 

hills 



Actual 



Grams 



Relative 



310 


3 


83.6 


465.8 


149 


3 


87.0 


478.2 


44 


3 


86.3 


490.3 


132 


3 


88.0 


526.6 


57 


3 


91.0 


666.5 



288 


3 


95 


509 


211 


3 


96 


519 


258 


3 


102 


555 


234 


3 


99 


585 


198 


3 


101 


631 



Per cent 

100 
103 
105 
113 
143 

100 
102 
109 

115 
125 



RELATION OF STAND TO YIELD IN SINGLE-ROW TEST PLATS 

The data in Table 22 were compiled from records of exten- 
sive ear-to-row tests of Hogue's Yellow Dent corn made by 
Lyon and Montgomery at the Nebraska Station during the 
four years 1904-1907. Rows 72 hills in length had been 
planted by hand at the rate of three kernels per hill, 3 feet 8 
inches apart. The entire plats were harvested regardless of 
the actual stand secured, altho a record was taken of the per 
cent stand. 

In Table 22 the plat yields have been assembled into groups 
for each year according to the per cent stand. Since a rather 
large number of plats are averaged in each group, this may 
overcome in large measure any inherent difference in yield- 
ing power of the individual ears tested, and the differ- 



44 Nebraska Agricultural Exp. Station, Research Bui. 13 



Table 22 — Relation of per cent germination in the field to 
yield of single-rotv test plats of Rogue's Yellow Dent corn 
(1904-1907) 



Year 



Number 
of plats 
averaged 



Kernels 
planted 
p?r hill 



Average 
field 
germi- 
nation 



Per cent 



GERMINATION 00-05 PER CENT 



■00 PER CENT 



S5 PER CENT 



GERMINATION 75-80 PER CENT 



GERMINATION 70-75 PER CENT 



-70 PER CENT 



GERMINATION BELOW 60 PER CENT 



Yield per 
acre 



Bushels 



1904 


10 


3 


92.1 


76.8 


1905 


9 


3 


92.3 


94.6 


1906 


2 


3 


93.0 


84.8 


1907 


22 


3 


94.0 


85.9 


Average 


43 


3 


92.8 


85.5 



1904 


12 


3 


87.6 


81.3 


1905 


25 


3 


88.1 


95.2 


1906 


10 


3 


87.0 


92.4 


1907 


16 


3 


86.0 


83.7 


Average 


63. 


3 


87.2 


88.1 



1904 


27 


3 


83.1 


75.4 


1905 


40 


3 


83.2 


88.4 


1906 


32 


3 


82.6 


85.4 


1907 


18 


3 


82.0 


85.0 


Average 


117 


3 


82.7 


83.5 



1904 


12 
14 
18 
16 


3 
3 
3 
3 


78.0 
78.4 
78.0 
77.0 


76.2 


1905 


85.5 


1906 


83.3 


1907 


83.9 


Average 


60. 


3 


77.8 


82.2 



1904 


11 

6 

19 

10 


3 
3 
3 
3 


74.0 
73.2 
73.4 
72.0 


68 1 


1905 


79 9 


1906 


82 9 


1907 


80 6 






Average 


46. 


3 


73.1 


77.9 



1904 


13 
3 

10 
10 


3 
3 
3 
3 


66.2 
67.3 
68.1 
65.0 


67 3 


1905 


77 3 


1906 


80 1 


1907 


74.7 


Average 


36. 


3 


66.6 


74.8 



1904 


21 

6 
11 

7 


3 
3 
3 
3 


35.6 
51.5 
42.1 
43.0 


42 6 


1905 


70 7 


1906 


56.9 
56.8 


1907 


Average 


45 


3 


43.0 


56.7 



Experimental Error in Crop Tests 45 

ence in yield for the groups may be assigned primarily 
to the difference in stand. During the four years, consid- 
ering three plants per hill a 100 per cent stand, stands aver- 
aging 92.8, 87.2, 82.7, 77.8, 73.1, 66.6, and 43.0 per cent 
yielded respectively 85.5, 88.1, 83.5, 82.2, 77.9, 74.8, and 56.7 
bushels per acre. 

It appears from these results that what was regarded a 
perfect stand, namely three plants per hill, was too thick for 
a maximum yield with this variety, since an 87.2 per cent 
stand outyielded a 92.8 per cent stand. The yield by no means 
decreased in proportion to the stand. An average stand of 
43 per cent yielded 66.3 per cent as much as a 92.8 per cent 
stand. It would appear unreliable to correct yields upon a 
basis of stand. 

The yield of an individual row plat planted at a given 
rate will vary greatly according to the stand in adjacent 
rows. For this reason the data in Table 22 must not be 
regarded as necessarily indicating the true relative yields, 
during the years tested, for the different stands as would 
be obtained in a proper rate-of-planting test. 

Because of the chance variations in stand of single-row 
plats, no reliable formulas can be established for the correc- 
tion of yields according to the per cent stand. For example, 
very different results may be expected from a row with 75 
per cent stand, according to whether it falls between rows 
having a 50 per cent or a 100 per cent stand. This is borne 
out by the rate-of-planting tests in rows and blocks during 
the three years 1914-1916 (Tables 11, 12, and 13). 

COMBINATION OF RATE-PLANTING AND VARIETY YIELD TESTS 

It has been a rather common practice in variety yield 
tests to plant all varieties at one arbitrary "standard" rate, 
regardless of their growth habits. 

During 1907 and 1908, three varieties were tested at five 
different rates of planting. The Pride of the North and 
Calico, which are respectively small and medium-sized vari- 
eties, increased regularly in yield with the rate of planting, 
and produced their maximum at the rate of five plants per 
hill. On the other hand, Mammoth White Pearl, which is _a 
large late corn, yielded its maximum at the three-rate and 
then fell off sharply. 



46 Nebraska Agricultural Exp. Station, Research Bui. 13 

In 1914, three varieties, differing distinctly in size and 
length of growing season, were planted at five different rates. 
Pride of the North produced its maximum yield at the rate 
of five plants per hill. University No. 3 produced identical 
and maximum yields at both the two and the three-rate and 
then fell off sharply. Hogue's Yellow Dent produced its max- 
imum yield at the two-rate and then fell off sharply. 

The data in both Tables 23 and Table 24 indicate that 
the relative yielding power of varieties differing in growth 
habit can only be determined by planting at several rates. 
Different varieties have a different optimum rate of planting. 



Table 23 — Relation of rate of planting to yield of corn varie- 
ties differing in growth habit grown in two-row plats* 
(1907-1908) 



Plants per hill 


Length 
growing 
period 


Yield per acre 


1907 


1908 


Average 




Days 


Bushels 


Bushels 


Bushels 



PRIDE OF THE NORTH 



MAMMOTH WHITE PEARL 



135 
135 
134 
133 
133 



45.6 
59.1 
70.7 
52.0 
61.1 



43.8 
65.6 
71.9 
59.4 
56.2 



1 


127 


33.7 


25.0 


29.3 


2 


126 


48.2 


37.5 


42.8 


3 


126 


55.3 


45.5 


50.0 


4 


125 


63.8 


51.6 


57.7 


5 


125 
CALICX 


69.4 
) 


48.4 


58.9 


1 


127 


43.1 


28.1 


35.6 


2 


123 


53.4 


40.6 


47.0 


3 


126 


71.0 


53.1 


62.0 


4 


125 


74.8 


56.2 


65.5 


5 


124 


78.7 


64.1 


71.4 



44.7 
62.3 
71.3 
55.7 
58.6 



"Plats not duplicated. 



Experimental Error in Crop Tests 



47 



EFFECT OF REMOVING SUCKERS WITH DIFFERENT VARIETIES 

Occasionally an investigator has removed the suckers from 
his corn varieties or selections in order to avoid annoyance 
by them. The data in Table 25 indicate that the removal 
of suckers may affect different varieties differently, and that 
a new error in testing may be introduced thereby. 

Table 24 — Relation of rate of planting to yield of corn varie- 
ties differing in growth habit grown in three-row plats 
(191 V 



Plants 
per 
hill 



No. of 
replica- 
tions 



1 


3 


2 


3 


3 


3 


4 


3 


5 


3 


1 


3 


2 


3 


3 


3 


4 


3 


5 


3 



Length 
growing 
Period 



Barren 
stalks 



Two- 
eared 

stalks 



Days Per cent Per cent 

PRIDE OF THE NORTH 



No. of ear 
bearing 
suckers 
per 100 
plants 



Yield per 

acre 

(center 

row) 



Bushels 



92 
92 
92 
92 
92 



107 
107 
107 
107 
107 



UNIVERSITY NO. 3 



8 


7 


17.4 


1 


2 


28.2 








35.5 








39.8 








44.4 





1 

6 

8 
15 



14 
3 
1 





20 
2 






40.2 
59.6 
59.5 

52.7 
47.3 



HOGUE'S YELLOW DENT 



1 


3 


119 





10 


19 


44.4 


2 


3 


119 


1 


1 


2 


63.9 


3 


3 


119 


2 








59.0 


4 


3 


119 


7 








59.8 


5 


3 


119 


13 








53.7 



RELIABILITY OF ESTIMATING PLAT YIELDS BY MEANS 
OF FRACTIONAL AREAS 



In conducting field experiments in cooperation with 
farmers, experiment stations frequently encounter difficulty 
in having test plats properly harvested and threshed. In some 
states the yields of such plats are estimated by harvesting 
a number of very small apparently representative areas 
from each of the plats to be compared. The small quantity 



48 Nebraska Agricultural Ex%>. Station, Research Bui. 13 



Table 25 — Effect of removing tillers from corn varieties dif- 
fering in growth habits (1912 and 1911+) 



Pride of the North. . . 

University No. 3 

Hogue's Yellow Dent . 
Pride of the North . . . 

University No. 3 

Hogue's Yellow Dent. 





Plants 
per 
hill 


No. of 


Yield per acre* 


Variety 


replica- 
tions 


Tillers 
on 


Tillers 
removed 


Differ- 
ence 








Bushels 


Bushels 


Bushels 



YEAR 


1912 






2 


10 


38.6 


30.9 


2 


10 


47.7 


42.9 


2 


10 


53.7 


43.5 


3 


10 


40.9 


38.2 


3 


10 


56.9 


54.2 


3 


10 


43.6 


38.8 



Pride of the North 

University No. 3 

Hogue's Yellow Dent 

Pride of the North 

University No. 3 

Hogue's Yellow Dent. . . . 

* Yield per acre based on center row of three- 
single-row plats in 1 it 1:2. 



YEAR 


1914 






2 


3 


35.3 


32.5 


2 


3 


49.2 


50.5 


2 


3 


52.3 


55.0 


3 


3 


38.8 


33.6 


3 


3 


45.8 


46.6 


3 


3 


54.4 


54.3 



7.7 
48 
10 2 
2.7 
2.7 
4.8 



2.8 
+ 1.3 
+2.7 

5.2 
+0.8 

0.1 



row plats in 1914 and on 



of grain harvested in this manner can readily be shipped to 
the central station for threshing and estimation of yield. In 
order to secure information relative to the reliability of such 
a method the following test was made in 1917 : 

Duplicate thirtieth-acre field plats of each of seven differ- 
ent varieties or selections of winter wheat were chosen from 
among a large number of plats for this study. These plats 
measured 16 rods by 66 inches and contained eight rows. 

Twenty systematically distributed fractional areas or 
quadrates were harvested from each plat. These were 32 
inches square, contained four rows of wheat, and were 
.0001632 acre in area. Quadrates were located 10 feet from 
each end and at intervals of 14 feet on alternate sides of the 
plat, as indicated in the following diagram. 



m s [u &r~ " — H — R — 



Diagram showing distribution of 2 quadrates in 
thirtieth-acre plats (Table 26) 



Experimental Error in Crop Tests 



49 



The quadrates were accurately laid out by means of an 
iron frame, as shown in the following figure. A rectangular 
frame is more reliable than a round one where the grain is 
planted in rows. 




Frame used for laying off quadrates (Table 26) 



Because of severe and variable winterkilling the 14 plats 
differed markedly in the percentage of plants surviving, and 
in yield. There was also much greater variation between 
the quadrates within a single plat than would normally be 
expected. 

Opportunity was provided to compare the mean results 
of 5, 10, and 20 systematically distributed quadrates with 
the entire plat from which they were harvested. In making 
four groups of five quadrates each, group (a) contained quad- 
rates Nos. 1, 6, 9, 14, and 17; group (b) contained Nos. 3, 
8, 11, 16, and 19; group (c) contained Nos. 2, 5, 10, 13, and 
18; and group (d) Nos. 4, 7, 12, 15, and 20. For two groups 
of 10 quadrates each, group (a) contained Nos. 1, 4, 5, 8, 9, 
12, 13, 16, 17, and 20, and group (b) contained Nos. 2, 3, 6, 
7, 10, 11, 14, 15, 18, and 19. The results of these various 
groupings are shown in Table 26 in comparison with the yields 
of the entire respective plats. 

The average yield determined from 20 quadrates deviated 
1,4 bushels from the average plat yield. 

For individual plats the 20-quadrate yield estimation 
varied from 0.2 to 3.2 bushels per acre. 

Smce each kind of wheat was grown in duplicate plats 
the mean of 40 quadrates can be compared with the mean 
of two field plats. In this comparison the average of these 



50 Nebraska Agricultural Exp. Station, Research Bui. 13 



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Experimental Error in Crop Tests 



51 



quadrate means, for the several sorts of wheat, deviated 2.2 
per cent from the average of the duplicate plat yields. 

When the quadrates from each plat were grouped into 
sets of five and ten each, there was considerable variation 
in yield between the separate groups, which suggests that 
not less than 20 quadrates should be harvested from compara- 
tive plats of this character. 

It appears that the results from 20 systematically dis- 
tributed quadrates may be fairly safely substituted for the 
yield of the entire plat from which they are taken. 

EXPERIMENTAL ERRORS CAUSED BY SOIL VARIATION 

The lack of uniformly productive land for comparative 
crop tests has given rise to a number of methods frequently 
used for ascertaining and overcoming the resultant experi- 
mental error. Chief among these methods are : ( 1 ) The use of 
frequent, systematically distributed check plats planted to a 
uniform crop for the purpose of (a) indicating the degree 




Fig. 13 — A relatively uniform field containing 2 07 thirtieth-acre plats 
sown for a method study to a uniform crop of Kherson oats (1916) 



52 Nebraska Agricultural Exp. Station, Research Bui. 13 

of variation due to the soil or (b) correcting the results from 
the intervening - test plats. (2) Replication of plats and bas- 
ing the conclusions upon the mean yield. (3) Use of long, 
narrow rather than short, wide plats. (4) Calculating the 
probable error for the mean results of replicated plats, to 
indicate the degree of confidence which may be placed in the 
results. 

The results from 207 thirtieth-acre Kherson oats plats, 
grown in 1916, illustrate each of the four practices mentioned 
above. These plats were planted to a uniform crop upon a 
seemingly uniform field for the purpose of studying varia- 
tion in plat yields as a source of experimental error. The 




Fig. 14 — Two hundred and seven thirtieth-acre Kherson oats plats 
planted to a uniform crop for studying experimental error in 1916 

entire field had been cropped uniformly to silage corn for a 
period of eight years. It had been plowed each year and was 
also plowed in preparation for the oats in 1916. The oats 
were drilled during two successive days in plats 16 rods by 
66 inches, which equaled one drill width. The plats were 
separated by a space of 16 inches between outside drill rows. 
A wide discard border of oats was grown around the outer 
edge of the field, so that all plats should have a similar expo- 
sure. General views of this field are shown in Figures 13 
and 14. 



Experimental Error in Crop Tests 53 

USE OF CHECK PLATS 

During the past 15 years it has become the general prac- 
tice in crop investigations to plant check plats at regular 
stated intervals. These plats are planted to a uniform crop 
and should yield alike except for various environmental 
sources of experimental error. 

The use of check plats may be twofold: (1) To indicate 
the error caused by variation in normal plat yields. The 
variation in the check plats is regarded as indicative of the 
error in the test plats. (2) Check plats are more commonly 
used to calculate the normal or theoretical yield of all plats in 
the field. All crops or treatments are then compared directly 
with each other by their increased or decreased yield above 
or below the calculated normal yield for the plats upon which 
they grew. This difference is best expressed in percentage 
of the normal plat yield. Comparative yields per acre may 
then be calculated for each crop, variety, or treatment by 
adding (or subtracting) the difference between it and the 
normal yield for the plat to (or from) the mean yield for all 
check plats in the field. This recalculation of yields is usually 
spoken of as correction according to check plats. 

The check plats may be variously distributed in the field 
according to the manner in which the corrections are to be 
made. Three methods of correction are in common use: (1) 
The normal or theoretical yield of the test plat is determined 
by, and is equivalent to, the average of two adjacent check 
plats. (Alternating plats are check plats.) (2) The normal 
or theoretical yield of the test plat is determined by, and is 
equivalent to, the yield of a single adjacent check plat. (Two 
test plats are planted between checks.) (3) The soil between 
two or more check plats is regarded as varying gradually 
from one check plat to the other and a progressive correction 
is used to establish the normal or theoretical yields of the 
intervening test plats. Thus, if two test plats lie between 
checks which yield 51 and 60 bushels respectively, the nor- 
mal yields assigned to the two test plats by this progressive 
method would be 54 and 57 bushels. Progressing from the 
lower to the higher yielding check the normal yield of the 
first test plat is greater than the poorer check by one-third 
of the difference, while the normal yield of the second test 
plat is greater than the poorer check by two-thirds of the 
difference. The proportion of the difference added to each 
successive test plat will depend upon the number of plats be- 
tween checks. 



54 Nebraska Agricultural Exp. Station, Research Bui. 13 



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57 



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59 



60 Nebraska Agricultural Exp. Station, Research Bui. 13 

The three foregoing tables (27-29) show the exact ar- 
rangement in which the 207 Kherson oats plats were grown 
in the field. Certain plats have been designated as check 
plats according to each of the above three methods, and the 
intervening plats have been treated as test plats. The test 
plats have been corrected in yield according to the check 
plats. If such correction had been effective, the coefficient 
of variability for the corrected yields would have been ma- 
terially reduced below the coefficient of variability for the 
actual yields. On the contrary, however, the coefficients of 
variability were reduced less than 1 per cent, being 7.8 per 
cent for the actual yields and 7.0 per cent for the corrected 
yields, as an average for the three methods of correction. 

Table 30 gives the coefficients of variability for the actual 
and corrected vields of the test plats indicated in Tables 27, 
28, and 29. 



Table 30 — Effect upon yield from correcting thirtieth-acre 
Kherson oats field plats according to various accepted 
means of check plat correction* (1916) 



Arrangement of 

check plats used 

for correction 


Fre- 
quency 


Intervening 
plat yields 


Standard devia- 
tion from 
mean for 


Coefficient of 
variability for 


Actual 
yields 


Cor- 
rected 
yields 


Actual 
yields 


Cor- 
rected 
yields 


Actual 
yields 


Cor- 
rected 
yields 


Alternate check 
plats. Correc- 
tion based upon 
average of two 
adjacent checks 

Checks every third 
plat. Correc- 
tion based upon 
one adjacent 
check plat . ... 

Checks every third 
plat. Correc- 
tion by progres- 
s i v e method 
based upon two 
nearest checks . . 


102 
138 

132 


Bushels 

78.2 

78.0 
78.0 


Bushels 
78.1 

77.7 
77.7 


Bushels 
6.14 

6.08 
6.13 


Bushels 
5.47 

5.71 
5.10 


Per cent 

7.85 

7.79 
7.87 


Per cent 
7.01 

7.35 
6.57 



♦Calculated from data in Tables 27, 28, and 29. 



Experimental Error in Crop Tests 



61 



REDUCTION OF ERROR BY REPLICATION 

The actual yields from the first 200 of these similarly 
treated plats of Kherson oats, described on pages 52 to 60, 
have been compiled to show the extreme variations, average 
and standard deviations from the mean, and the coefficients 
of variability for single plats and for the mean yields of two, 
four, and eight plats averaged together. These groupings 
have been arranged for both adjacent and systematically dis- 
tributed plats. The results are given in Table 31. 

It is clearly shown that replication greatly reduces the 
extreme variation and coefficient of variability in the yield 
of field plats. A given number of replications are also much 
more effective when systematically distributed than when 
adjacent plats are averaged. 




Fig. 15 — Harvesting thirtieth-acre plats of Kherson oats. The binder 
has a gasoline engine attached which cuts and binds the grain. This 
facilitates cleaning out the binder quickly at the end of each plat. 
Note the narrow bare spaces between plats. If the plats are tangled 
by lodging, they are separated by hand before being cut. This 
shape of plat is very convenient, since it is one drill in width and 
may be harvested by one swath of the binder 



62 Nebraska Agricultural Exp. Station, Research Bui. 13 



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64 Nebraska Agricultural Exp. Station, Research Bui. 13 

The yield of the 200 individual plats varied from 56.7 to 
92.8 bushels per acre. The mean for eight groups of 25 
single plats each gives an extreme difference between single 
plats of 20.7 bushels per acre. When two, four, and eight 
systematically distributed plats are averaged, the extreme 
differences in yield are respectively 14.9, 8.9, and 7.8 bushels. 
When two, four, and eight adjacent plats are averaged, these 
extreme differences are 19, 16.7, and 15 bushels. For sys- 
tematically distributed plats the coefficients of variability for 
one, two, four, and eight plats in a group are 6.30, 4.5D, 
2.91, and 2.18 per cent. For adjacent plats the coefficients 
of variabilitv for one, two, four, and eight plats in a group 
are 6.30, 5.46, 5.28, and 4.78 per cent. 

Systematic distribution of replicated plats is seen to be 
very effective in reducing experimental error due to environ- 
mental variations. 

EFFECT OF SHAPE AND SIZE OF l'1-AT 

The 207 thirtieth-acre Kherson oats plats described in 
the preceding discussion were grouped to enable a compari- 



Various ways of combining plats to make plats of 
different sizes and shapes (Table 31) 



Experimental Error in Crop Tests 65 

son of long narrow plats with short wide plats. The group- 
ings illustrated in the following diagrams were compared. 
(In the 1x9 grouping, three groups were necessarily irregu- 
lar in shape since 9 is not a multiple of 69.) 

The results are included in Table 31. Long, narrow plats 
are indicated to be more reliable than short wide plats of 
the same area. Increasing the size of the plat is less effec- 
tive in overcoming experimental error than the systematic 
distribution of plats equal in combined area. 

SIGNIFICANCE OF THE "PROBABLE ERROR" 

The "probable error" calculation is being used somewhat 
by field crop experimenters. Its use is rather inviting since 
a small "probable error" is customarily regarded as indicat- 
ing accuracy in the results. Davenport's interpretation is 
generally accepted, namely: "It (the probable error) indi- 
cates the degree of confidence which we should place in results 
obtained by statistical methods." 

Where plats are replicated two or more times, the prob- 
able error of the mean, is based upon the standard deviation, 
and is determined by the following formula : 

standard deviation 
- Probable error of mean = ± 0.6745 — === ===== . , 

l number of vanates 

a 

which is also stated E: m — ± 0.6745 — t= 

1 n 

The probable error is regarded as an upper and lower 
limit of divergence for which the chance is even that the 
true mean does not lie outside of these limits. Commenting 
upon the likelihood of the true mean lying outside of the 
limits set by the probable error, Davenport (1907) states: 

"Of course the error in a determination has also an even 
chance of lying outside the limits set by the probable error 
(E) , but the following table will show that it is very unlikely 
that the error is many times as great as E. Thus the chances 
that the true value lies within the range set by ± E, ± 2E, 
etc., are as follows : 



66 Nebraska Agricultural Exp. Station, Research Bui. 13 

± E the chances are even 

±2 E the chances are 4.5 to 1 

±3 E the chances are 21 to 1 

±4 E the chances are 142 to 1 

±5 E the chances are 1310 to 1 

±6 E the chances are 19,200 to 1 

±7 E the chances are 420,000 to 1 

±8 E the chances are 17,000,000 to 1 

±9 E the chances are about 1,000,000,000 to 1 

"It is extremely improbable, therefore, that an error will 
be many times as large as the probable error. For instance, 
it is practically certain that the error is not as large as 9 E, 
since the table shows that the chances are about a billion to 
one in favor of its being smaller than 9 E. 

"Thus by giving, along with any result, the calculated 
probable error, the reader may know what degree of con- 
fidence is to be placed in the results." 

In common usage, it is stated that the actual difference 
in the yield of two plats must be three times the probable 
error before the difference in yield is significant. 

It should be agreed at the outset that the probable error 
of a mean yield has significance only when the variations 
entering into the mean are purely accidental rather than sys- 
tematic. This distinction is understood by biometricians 
who universally attach importance to the probable error cal- 
culation when used in a legitimate manner. There appear to 
be strong possibilities of misusing the probable error and 
overestimating its value in agronomic studies. This need not 
be regarded as any defect in the probable error formula, but 
rather as a misapplication thereof to experimental results 
possessing either visible or invisible systematic errors. 

Field crop investigators consider it good technique to repli- 
cate test plats. It has been proposed that, in such tests, small 
probable errors for the mean yields of the various varieties 
or treatments would indicate reliability and justify con- 
fidence in the comparative yields. 

For the purpose of studying the significance of the prob- 
able error in field crop tests, the first 200 consecutive thir- 
tieth-acre Kherson oats plats described on pp. 52 to 64 have 
been grouped in 50 sets of four adjacent plats and also 50 
sets of four systematically distributed plats, and the prob- 
able error calculated for the mean yield of each group of 
four plats. 



Experimental Error in Crop Tests 67 

PROBABLE ERROR FOR FIFT\' GROUPS OF FOUR ADJACENT 
THIRTIETH-ACRE PLATS OF KHERSON OATS 

That the probable error cannot apply to the mean yields 
of adjacent duplicate plats in a variety test is brought out 
by the following data : 

In Table 32 are given the mean yields for 50 groups of 
four adjacent plats, together with the average deviation, 
standard deviation, and probable error for each group. The 
average deviation of each group from the mean yield for the 
entire 200 plats is also indicated and in the last column of 
the table is given the ratio of this deviation to the probable 
error. 

If it is permissible to assume that one group of four dupli- 
cate plats is comparable with another group of four plats in 
the same field, then it would also seem permissible to assume 
that in the present instances, the mean yield for the entire 
200 similarly treated oats plats should represent the correct 
yield or true value of any or all of the individual groups 
within the field. If this assumption be made with the adja- 
cent duplicate plats (Table 32), the actual error of these 
group means exceeded their probable error approximately 0, 
1, 2, 3, 4, 5, 6, 7, 8, 10, 11, and 15 times respectively in 9, 5, 
7, 7, 8, 4, 4, 1, 2, 1, 1, and 1 groups. (See Col. 11, Table 32). 
This is very inconsistent with the table of probabilities quoted 
from Davenport on page 66, and shows that a uniform ap- 
pearing field may be so heterogeneous in soil conditions that 
its mean yield cannot be regarded as correctly representing 
the true value of its various parts. 

Since all the plats were treated and planted alike any dif- 
ference in the yields of the groups represents experimental 
error, either in mechanical operations or in soil variation. 

Among the 50 groups of adjacent plats, one group yielded 
14.2 bushels less and another group 7.3 bushels more per acre 
than the 200-plat mean. These extremes represent an experi- 
mental error of 21.5 bushels since both should have yielded 
alike if the method of comparison were reliable. 

Should we presume that groups No. 30 and No. 50 (Table 
32) are distinct varieties in a comparative variety test, we 
would have a difference in yield of 21.5 bushels per acre. 
After multiplying the probable error of each mean by three, 
there remains a net difference of 11.63 bushels between the 
probable error ranges. Placing confidence in the probable 
error calculation, we would believe that there is a difference 



68 Nebraska Agricultural Exp. Station, Research Bui. 13 






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Experimental Error in Crop Tests 



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72 Nebraska Agricultural Exp. Station, Research Bui. 13 

of 11.63 bushels in the true value of the two varieties. How- 
ever, we know in this case that both groups should have 
yielded alike since they were planted to the same crop. The 
probable error would give us confidence in very inaccurate 
results. 

Slightly different results are obtained when the above ex- 
ample is calculated by the following prescribed formula : "The 
probable error of the difference of two means each affected 
with a probable error, is equal to the square root of the sum 
of the squares of the probable errors." By this formula the 
difference in mean yield of groups Nos. 30 and 50 equals 
21.5 ±2.55 bushels. Three times the probable error is 7.65 
bushels which leaves a net difference of 13.85 bushels. 

PROBABLE ERROR OF FIFTY GROUPS OF FOUR SYSTEMATICALLY 
DISTRIBUTED THIRTIETH-ACRE PLATS OF KHERSON OATS 

Table 33 contains results with the same 200 Kherson Oats 
plats as compiled in Table 32, except that systematically dis- 
tributed plats rather than adjacent plats are averaged in 
groups of four each. If the mean yield of the entire 200 plats 
is here regarded as the true value of the various group means, 
the actual error of these group means exceeded their prob- 
able error 0, 1, 2, 3, and 4 times in 10, 25, 10, 1, and 4 groups 
(See Col 11). This is a marked reduction in actual error 
as compared with similar data for adjacent plats and indi- 
cates a great advantage for systematic distribution. An ap- 
plication of the probable error to these systematically distrib- 
uted plats would seem fairly reasonable altho it cannot be 
applied absolutely. 

Because of chance groupings of either large or small varia- 
tions where relatively small numbers are used, the actual 
error of a mean may be greater than three times its probable 
error, or it may be smaller than the probable error. Data may 
be either more or less accurate than an application of the 
probable error would indicate. 

EXAMPLES OP LIMITATION OF THE PROBABLE ERROR 

Small Grain Roiv Tests — In Tables 1 to 7 were given the 
relative small grain yields of rate-of-planting or variety tests 
in alternating nursery rows. The plats were replicated 50 
times and the probable error of the mean yields is indicated. 
The yields in these plats were subject to two sources of error, 
namely soil variation and plat competition. Corresponding 



Experimental Error in Crop Tests 73 

tests were also made in five-row plats relatively free from 
plat competition and subject primarily only to soil variations. 
In Table 1 (1913) the yields of the thick and thin planted 
wheat rows were, respectively, 389±5.3 and 264±3.8 grams. 
Altho the probable error for each yield is less than 2 per cent, 
the actual error of the relative yields due to competition is 
24.4 per cent. In 1914 the yields of the thick and thin planted 
wheat rows were respectively 327 ±6.66 and 115 ±3.6 grams. 
Altho the probable error for each yield is only 2 per cent, the 
actual error of the relative yields, due to competition, is 56.8 
per cent. 

In 1913 (Table 2) the probable errors for the mean yields 
of thick and thin planted oats rows were less than 2 per 
cent, but the actual error in relative yields, due to competi- 
tion, was 20 per cent. In 1914 the probable errors for simi- 
lar yields were also below 2 per cent, while the actual error 
in relative yields, due to competition, was 34.3 per cent. 

Similar examples are seen in variety tests in Tables 3 to 
7. We would have great confidence in these single-row tests 
were we to judge them by their low "probable errors." How- 
ever, it is evident that this confidence would be badly mis- 
placed. 

Crop tests are subject to such a multitude of local environ- 
mental influences that errors in them cannot be regarded as 
occurring according to the formulas or rules of chance cal- 
culated from purely mechanical observations. The probable 
error calculation may apply, for example, to the chance draw- 
ing of black and white marbles from a bag at a given ratio 
to each other. But variations in crop yields are no such sim- 
ple matter, and the probable error not only may have little 
significance but may be misleading. 

Water Requirements of Corn and Wheat — As further illus- 
tration of the limitation of the probable error, the following 
simple data from our 1916 water requirements of crop studies 
may be cited. 

The object was to make a comparative test of the relative 
water requirements for grain production of a standard variety 
of both corn and winter wheat. Potometers, 16 by 36 inches 
in size and containing 250 pounds of well-manured moist- 
ure-free soil, were used. (The method of testing is de- 
scribed in detail in Nebraska Research Bulletin No. 6.) 

Previous experiments had indicated that these potometers 
would grow one corn plant in a normal manner. The ratio 



74 Nebraska Agricultural Exp. Station, Research Bui. 13 

of 100 seeds of wheat to one of corn is normal in planting 
under field conditions in this region. Accordingly in com- 
paring corn and wheat in potometers they were planted re- 
spectively at the rates of one plant and 100 plants per pot. 

Under these conditions the respective water requirements 
for grain production of the corn and wheat were 743 ±48 
and 1017 ±60. However, when the corn was grown at the 
rate of six plants per potometer these relative water require- 
ments were 3481 ±389 and 1017 ±60. 

Applying the general rule of "three times the probable 
error," we may be fairly confident from the one comparison 
that Hogue's Yellow Dent corn uses considerably less water 
than Turkey Red winter wheat, and from the other compari- 
son we may be equally confident that corn uses more than 
double the amount of water for grain production than the 
wheat. 

In the first comparison the degree of cropping for this 
quantity of soil corresponded well with normal field conditions 
for each crop. In the second test, however, the corn was 
planted relatively much too thick, and for this reason the 
ratio of grain to vegetative growth was greatly reduced. As 
a result the water requirement for grain production was in- 
creased. 

EFFECT OF CHANGE IN METHODS ON AGRONOMIC EQUIPMENT 

Replacing the single-row nursery test plat planted in du- 
plicate with five-row test plats replicated 10 times increases 
the land requirement 25 times for such nursery testing. In 
testing hoed crops the substitution of three-row plats, repli- 
cated five times, for single duplicated rows requires 15 rows 
rather than two rows. The replication of small grain field 
plats five times, rather than twice, greatly increases the land 
requirement. 

Fertilizer and tillage experiments which frequently are 
conducted in unduplicated plats should probably be at least 
triplicated. Reduction of error by replication is more effec- 
tive than the use of check plats alone. 

The introduction of check plats every fifth plat in itself 
occupies one-fifth of the land. The more refined methods of 
securing comparable stands of corn upon which to base the 
yields at harvest require much greater labor expenditure than 
formerly. 



Experimental Error in Crop Tests 



75 



The proper conduct of experimental work in crop produc- 
tion in light of our present knowledge requires either a large 
extension in land area and labor facilities or else a marked 
restriction in the amount of investigation carried on. 

MEASURING IMPROVEMENT i:, T YIELD THRU BREEDING 

Comparing the yield of corn for one period of years with 
the yield of another period is an unreliable method for not- 
ing improvement thru corn breeding. An illustration of this 
method is found in a circular of the United States Depart- 
ment of Agriculture Office of Corn Investigations, August 20, 
1914. The data in Table 34 were given in this circular as 



Table 34 — Data given in Circular of Office of Corn Investiga- 
tions, U. S. Department of Agriculture, August 20, 1914, 
to show improvement from ear-to-row breeding conducted 
at Piketon, Pike County, Ohio 



Yield per acre as weighed in the fall 
(70 lbs. of ears to the bushel) .... 

Yield per acre of dry shelled grain 
(56 lbs. to the bushel) 



Average for Average for R .. fi . 
first seven second seven ■ _, , 

years, 1901- ' years, 1907- ge S period 
1907 inclusive 1913 inclusive seconcl periocl 



Bushels 


Bushels 




77 


85 


100:110.4 


63 


75 


100:119 



indicating 19 per cent increase in yield of dry shelled corn 
per acre by ear-to-row breeding. The increase in yield of 
ear corn as weighed at husking time was 10.4 per cent. The 
measure of improvement by breeding was the average in- 
creased yield during a seven year period, 1907-1913, over the 
previous seven-year period. 

A comparison of the yields in Table 35 during these same 
two periods for the state of Ohio as compiled from the United 
States Yearbook indicates a similar increase in yield for the 
state in general. During the last period of seven years, the 
Ohio state yield was 11.4 per cent higher than during the 
previous seven years. Likewise data compiled from the re- 
ports of the Ohio State Secretary of Agriculture, indicate 9.4 
per cent greater yield for Pike County, in which the experi- 
ments were conducted, during the last seven years than dur- 
ing the previous seven years. This suggests that more favor- 



76 Nebraska Agricultural Exp. Station, Research Bui. 13 



Table 35 — Ohio state and Pike County yields of corn averaged 
for the same periods as given in Circular of the office of 
Corn Investigations, August 20, 19 1U 



Yield per acre for 
state of Ohio as 
compiled from U. 
S. Yearbook 

Yield per acre for 
Pike County, Ohio, 
as compiled from 
the reports of the 
Ohio State Board 
of Agriculture 



Average for : Average for 
first seven second seven 
years, 1901- years, 1907- 
1907 inclusive 1913 inclusive 



Bushels 



34.4 



28.7 



Bushels 



38.3 



31.4 



Ratio first 

period to 

second period 



100:111.4 



100:109.4 



Average 

yield for 

nine years 

previous to 

first period 



Bushels 



32.8 



able climatic conditions may have been the cause of the appar- 
ent improvement of the ear-to-row corn. 

A similar method of measuring improvement by ear-to- 
row corn breeding at the Nebraska Experiment Station dur- 
ing the same period of 13 years, gives the results shown in 
Table 36. The yield of continuous ear-to-row breeding 
strains during the seven-year period 1907-1913 was 61 per 
cent as great as during the preceding seven years. It would 
appear that the corn yield had been reduced 39 per cent by 
ear-to-row breeding during the last seven years. However, 
a comparison of yields in Lancaster County, in which the 
Station is located, shows a decreased yield of 30 per cent, 
and the State as a whole a decreased yield of 17.3 per cent 
for the same two periods. Further, the yield of the original 
unselected Hogue's Yellow Dent corn showed a decreased 
yield of 35 per cent at the Experiment Station during the 
second seven-year period. All indications are that the reduced 
yield of ear-to-row corn at the Experiment Station was due 
to climatic conditions and not to the breeding. An actual 
comparison of the ear-to-row corn during the last period of 
seven years with the original corn of the same variety 
planted each year as a check indicates an actual increased 
yield of 5.4 per cent due to breeding, whereas the other 
method of comparison indicated a decreased yield of 39 per 
cent. 



Experimental Error in Crop Tests 



11 



TABLE 36 — Nebraska data compiled to show results secured by 
the Nebraska Experiment Station from ear-to-row breed- 
ing if compared by the method of the Office of Corn Inves- 
tigations reported in Table 31 





Average 


Average 




Average 




yield for 


yield for 




yield for 




first seven 


second seven 


Ratio 


nine years 




years, 1901- 


years, 1907- 




previous to 




1907 inclusive 


1913 inclusive 




first period 




Bushels 


Bushels 




Bushels 


Yield for State of Ne- 










braska as compiled 










from U. S. Year- 










book 


28.3 


23.4 


100:82.7 


24.1 


Average yield for 






Lancaster County. 


30.0 


21.0 


100:70 




General crop of Rog- 










ue's Yellow Dent 










corn at the Nebras- 










ka Experiment Sta- 










tion 


69.6 


45.6 


100:65.5 




Yield per acre, at the 










Nebraska Experi- 










ment Station of 










Hogue's Yellow 










Dent corn which 










has undergone con- 










tinuous ear-to-row 










breeding since 1902 


81.5* 


49.9 


100:61.0 




Yield per acre at the 










Nebraska Experi- 










ment Station of or- 










iginal unselected 










Hogue's Yellow 










Dent corn used as 










check for measur- 










ing improvement 










from breeding!.. . . 




47.2 







included 



*The yield for ordinary Hogue's Yellow Dent Corn for 1901 
in this average. 

tAveraging together these data for the seven years 1909-1915 — during 
which period the precaution was taken to have strictly comparable results 
by thinning to a uniform stand and to reduce error by several replications — 
we have an average yield for the continuous ear-to-row breeding stock of 
49.2 bushels, and the comparable check yield is 48.9 bushels. 



78 Nebraska Agricultural Exp. Station, Research Bui. 13 

A comparison of the Hogue's Yellow Dent ear-to-row-selec- 
tion with the original unselected Hogue's Yellow Dent corn 
for the seven-year period 1900-1915 — during which time the 
precaution was taken to have strictly comparable results by 
thinning to a uniform stand, and to reduce error by several 
replications — indicates an increased yield of only six-tenths 
of one per cent due to the breeding. 

In order to measure progress in the improvement of corn 
thru breeding, it is necessary to compare the results each 
year with the original unselected corn. 

SOIL LIMITATION AS A SOURCE OF ERROR 
IN POT EXPERIMENTS 



The past discussions in this bulletin have dealt entirely 
with field experiments. Extensive use has also been made of 
pots filled with soil for comparing the yields of various crops 
and soil types, and for determining the fertilizer needs of dif- 
ferent soils and the water requirement of crops. A review of 
the literature indicates a marked lack of uniformity in the 
size of pots and rate of planting in them. 

Tables 37 to 47 contain the results from experiments con- 
ducted during three years, 1913-1915, bearing upon the effect 
of the size and rate of planting as sources of experimental 
error in pot tests. 

Galvanized iron pots were used, having a constant water 
supply from jars connected at the bottom. Rain was excluded 
by means of a closefitting cover about the stalk, and surface 
evaporation was reduced by means of a three-inch layer of 
gravel. All pots were planted each year from the same ear 
of Hogue's Yellow Dent corn. Suckers were removed as soon 

Table 37 — Summary showing the effect of the size of the pot 
upon the growth of corn. Hogue's Yellow Dent corn 
(1913) 



Size of 
pot 


Wt. of 
soil 
(moisture- 
free) 


No. of 

pots 

averaged 


Dry matter 


Total 
leaf-area 
per plant 


Height of 
stalk 


Ear 


Total 




Inches 
12x24... 
16x36... 
30x36... 


Pounds 

86 

245 

933 


4 

80 

4 


Grams 

28 

194 

311 


Grams 
165 
416 
599 


Sq. in. 

680 

1070 

1440 


Inches 
71 
89 

83 



Experimental Error in Crop Tests 



79 



as they started, so as to prevent variability in the number of 
stalks per pot. Thus uniform conditions were provided thru- 
out all pots except the one or two variable factors under obser- 
vation. The pots were located in trenches within a cornfield, 
with their tops level with the field. They were filled with 
fertile surface soil from the Experiment Station Farm. The 
manure which was used in half of the pots during 1914 and 
1915, as designated, was well-rotted sheep manure, and was 
thoroly mixed with the upper ten inches of soil. 

Table 38 — Summary of data showing the effect of the size of 
pot upon grciv'h of corn. Hogue's Yellow Dent corn 
(19U) 



Size of 
pot 


Moisture-free 
contents 


No. of 

pots 

averaged 


Dry matter 


Total 
leaf- 
area 
per plant 


Height 
of plant 


Soil 


Manure 


Ear 


Total 


Inches 
12x12 . . . 
12x12 . . . 
12x24 . . . 
12x24 . . . 
16x24 . . . 
16x24 . . . 
16x36. . . 
16x36 . . 
21x36. . . 
21x36. . . 
30x36 . . . 
30x36 . . . 


Pounds 
32.5 
32.5 
85 
85 

150 

150 

239 

239 

583 

583 

956 

956 


Pounds 
1.75 
1.75 
1.75 
1.75 
1.75 
1.75 


4 
4 
4 
4 
4 
4 
3 
8 
4 
4 
3 
4 


Grams 
10 
82 
63 

186 

108 

270 

242 

287 

299 

341 

405 

416 


Grams 
98 
269 
206 
402 
316 
535 
442 
558 
628 
708 
728 
781 


Sq. in. 
705 
1167 
1165 
1353 
1343 
1369 
1193 
1322 
1308 
1405 
1269 
1287 


Inches 
76 
102 
100 
106 
110 
112 
116 
114 
112 
114 
108 
114 



Table 39 — Showing in per cent the effect of increasing the 
size of pot. The results in the different sizes without 
manure are here expressed in per cent of the results in 
the smallest size ivithout manure. Hogue's Yellow Bent 
corn (191b)* 





\7t. of soil 


Dry matter 


Total 


Height of 
stalk 


Size of pot 


im jisture- - 




leaf-area 




kee) 


Ear 


Tctr.I 


per plant 


Inches 


Pounas 


Per cent 


Per cent 


Per cent 


Per cent 


12x12 


32.5 


100.0 


100.0 


100.0 


100.0 


12x24 


85.0 


632.5 


211.0 


165.2 


131.3 


16x24 


150.0 


1082.3 


324.1 


190.6 


144.7 


16x36 


239.0 


2417.0 


453.6 


169.3 


153.0 


21x36 


583.0 


2990.0 


643.8 


185.6 


147.4 


30x36 


956.0 


4046.7 


747.0 


180.0 


142.1 



! 'Data calculated from Table 38. 



80 Nebraska Agricultural Exp. Station, Research Bui. 13 

EFFECT OF THE SIZE OF POT UPON THE GROWTH OF CORX 

In 1913 individual plants of Hogue's Yellow Dent corn 
were grown in pots of three different sizes. The results are 
summarized in Table 37. In pots containing 86, 245, and 
933 pounds of soil, the average total dry matter harvested 
per pot was respectively 165, 416, and 599 grams, while the 
average weights of ear corn were 28, 194, and 311 grams. 

In 1914, six sizes of pots were used, which contained 32, 
85, 150, 239, 583, and 956 pounds of moisture-free soil. Four 
pots of each size were cropped without manure and four with 
manure. The results are summarized in Table 38. Table 39 
shows in percentage the effect upon yields of increasing the 
pot size. Using the crop harvested in the smallest pots with- 
out manure as 100 per cent, the yields of total dry matter for 
the other sizes without manure were respectively 211, 324.1, 
453.6, 643.8, and 747 per cent. The yields of ear corn were 
respectively 100, 632.5, 1082.3, 2417, 2990, and 4046.7 per 
cent. 

Table 40 shows in per cent the effect of applying a uniform 
rate of manure to the pots of different sizes in 1914. The 
yield with manure is expressed in per cent of the yield with- 
out manure for each size. 

Table 40 — Showing in per cent the effect of applying a uni- 
form rate of manure to pots of different sizes. The results 
wit J} manure are here expressed in per cent of the results 
without manure. Hogue's Yelloiv Dent corn (1914)* 



Size of pot 



Wt. of soil 

(mcisture- 
free) 



Dry matter 



Ear 



Total 



Total 
leaf-area 
per plant 



Height of 
stalk 



12x12 

12x24 

16x24 

16x36. 

21x36 

30x36 



Inches 



Pounds Per cent Per cent Per cent 



32.5 
85.0 
150.0 
239.0 
583.0 
956.0 



822.5 
293.6 
249 2 
118.9 
114.1 
102.9 



276.4 
195.3 
169 3 

126.1 
112.7 
107.2 



165.6 
116.2 
101.8 
110.7 
107.4 
101.4 



Inches 
133.5 
106.2 
101.3 
98.3 
101.8 
105.5 



*Data calculated from Table 38. 



Applying 1.75 pounds of moisture-free manure per pot 
increased the yields of total dry matter for the different sized 
pots respectively 176.4, 95.3, 69.3, 26.1, 12.7, and 7.2 per cent. 
Likewise, the manure increased the yields of grain per pot 



Experimental Error in Crop Tests 



81 




1 



6 



Fig. 16 — Representative plants of Hogue's Yellow Dent Corn grown 
one stalk per pot, in pots of different sizes, 1914. (Table 38) 
Each set contains a plant grown with and without manure. Pounds 
of soil per pot, left to right 1 — 32.5; 2 — 85; 3 — 150; 4 — 239; 
5 — 583; 6 — 956 



82 Nebraska Agricultural Exp. Station, Research Bui. IS 

respectively 722.5, 193.6, 149.2, 18.9, 14.1, and 2.9 per cent, 
according to the size of the pot. 

In the above experiment for 1914, the manure was applied 
on the individual plant basis. Assuming a normal stand of 
3556 hills, each containing 3 plants, an acre of corn has 
10,668 plants. One and seventy-five one hundredths pounds 
of moisture-free manure per plant would be at the rate of 
9.33 tons per acre. 

In 1915, the same six sizes of pots were used as in 1914, 
and contained respectively 36, 83, 161, 253, 561, and 920 
pounds of moisture-free soil. There were eight pots of each 
size, four of which were manured. Table 41 contains a sum- 
mary of the results. Table 42 shows in percentage the effect 
of increasing the pot size upon yield. 

Based upon the yield in the smallest pots, without ma- 
nure, the relative yields of drv matter for the respective sizes 
were 100, 150, 229.6, 355.6, 586, and 578.7 per cent. The 
relative yields of ear corn were respectively 100, 276.2, 819, 
1,647.5, 2,771.3, and 2,667 per cent. 

Table 43 shows in percentage the effects of applying, to 
the pots of different sizes, manure in amounts proportional 



Table 41 — Summary of data shoiving the effect of the size of 
the pot upon the growth of corn. Hogue's Yellow Dent 
corn (1915) 



Size of 


Moisture-free 
contents 


No. of 

pots 

averaged 


Dry matter 


Total 
leaf- 


Height 


pot 








area 
per plant 


of plant 


Soil 


Manure 


Ear 


Total 


Inches 


Pounds 


Pounds 




Grams 


Grams 


Sq. in. 


Inches 


12x12 . . . 


36 




4 


10.5 


108 


753 


71 


12x12 . . . 


36 .08 


4 


17.8 


107 


776 


80 


12x24 . . . 


83 


4 


29 


162 


1061 


98 


12x24. . . 


83 .18 


4 


30 


172 


1219 


102 


16x24 . . . 


161 


4 


86 


248 


1150 


109 


16x24 . . . 


161 .36 


4 


76 


273 


1238 


111 


16x36. . . 


. 253 1 


4 


173 


384 


1209 


114 


16x36 . . . 


253 .55 


4 


203 


456 


1266 


111 


24x36 . . . 


561 




3 


291 


633 


1323 


120 


24x36 . . . 


561 


1.25 


4 


366 


684 


1372 


116 


30x36. . . 


920 




4 


280 


625 


1226 


116 


30x36 . . . 


920 


2.00 


4 


331 


685 


1307 


112 



Experimental Error in Crop Tests 



83 




l 



7 8 9 10 1 



Fig. 17 — Crop harvested from pots of six different sizes, 1915 (Table 
41). One plant was grown per pot, with four pots of each size. 
Odd numbers without manure, even numbers with manure. (Ma- 
nure added in proportion to soil contents.) 

Pounds of soil, left to right: 1 and 2 — 920 lbs.; 3 and 4 — 5 61 lbs.; 5 
and 6 — 253 lbs.; 7 and 8 — 161 lbs.; 9 and 10 — 83 lbs.; 11 and 12 — 
36 lbs. 



84 Nebraska Agricultural Exp. Station, Research Bui. 13 

to the amount of soil. Two pounds of moisture-free manure 
were applied to the largest pots, while the amounts added 
to the other sizes were respectively 1.25, 0.55, 0.36, 0.18, 0.8 
pounds. Expressed in per cent of the yields without manure, 
the manured pots yielded 99.1, 106.2, 110.1, 118.8, 108, and 

109.6 per cent total dry matter, and 169.5, 103.5, 88.4, 117.3, 

125.7 and 118.2 per cent of ear corn. 

Table 42 — Showing in per cent the effect of increasing the 
size of the pot. The results in the different sized pots 
without manure are here expressed in per cent of the re- 
sults in the smallest pots without manure. Hogue's Yel- 
low Dent corn (1915)* 





Wt. of soil 


Dry matter 


Total 


Height of 
stalk 


Size of pot 


(moisture- 






leaf-area 




free ) 


Ear 


Total 


per plant 


Inches 


Pounds 


Per cent 


Per cent 


Per cent 


Per cent 


12x12 


36 


100.0 


100.0 


100.0 


100.0 


12x24 


83 


276.2 


150.0 


140.9 


138.0 


16x24 


161 


819.0 


229.6 


152.7 


153.5 


16x36 


253 


1647.5 


355.6 


160.6 


160.6 


21x36 


561 


2771.3 


586.1 


175.7 


169.0 


30x36 


920 


2667.0 


578.7 


162.8 


163.4 



"Data calculated from Table 11. 



Table 43 — Summary of data showing the effect of applying 
manure proportional to the amount of soil in pots of dif- 
ferent sizes. The results with manure are here expressed 
in per cent of the results without manure. Hogue's Yel- 
low Dent corn (1915)* 





Wt. of soil 


Dry matter 


Total 


Height of 
stalk 


Size of pot 


(moisture- 




leaf-area 




free) 


Ear 


Total 


per plant 


Inches 


Pounds 


Per c<>.nt 


Per cent 


Per cent 


Per cent 


12x12 


36 


169.5 


99.1 


103.1 


112.7 


12x24 


83 


103.5 


106.2 


114.9 


104.1 


16x24 


161 


88.4 


110.1 


107.7 


101.8 


16x36 


253 
561 


117.3 118.8 
125.7 108.0 


104.7 
103.7 


97.4 


24x36 


96.6 


30x36 


920 


118.2 109 6 


106.6 


96.6 



l);iia calculated from Table 41. 



Experimental Error in Crop Tests 



85 



EFFECT OF PLANTING AT DIFFERENT RATES UPON THE 
GROWTH OF CORN IN POTS 

In 1915, corn was planted at four different rates, namely 
one, two, four, and six plants in pots 16 by 36 inches in size 
and containing 253 pounds of soil. The results are contained 
in Tables 44, 45, and 46. Without manure (Table 45) the 
individual plants in the six, four and two-rate yielded respec- 

Table 44 — Summary of data showing the effect of different 
rates of planting upon growth of corn in pots. Hogue's 
Yellow Dent corn (1915) 













Total 




Rates of 
planting 
per pot 


Moisture-free 
contents 


No. pots 
averaged 


Dry matter* 


leaf- 
area 
per 


Height 
of stalk 














Soil 


Manure 




Ear 


Total 


plant f 






Pounds 


Pounds 




Grams 


Grams 


Sq. in. 


Inches 


1 


253 




4 


232 


476 


1334 


123 


1 


253 


1.55 


8 


262 


539 


1457 


115 


2 


253 




4 


92 


242 


1210 


120 


2 


253 


1.55 


4 


118 


279 


1153 


112 


4 


253 




4 


37 


127 


895 


106 


4 


253 


1.55 


4 


37 


151 


990 


105 


6 


253 




4 


6.5 


79.0 


714 


90 


6 


253 


1.55 


4 


' 16.7 


101.9 


861 


93 



*Where more than one plant was grown in a pot, the average yield 
per plant is given. 

tThe leaf-area is not very significant inasmuch as the lower leaves died 
prematurely according - to the rate of planting — due to malnutrition. 



Table 45 — Summary of data shoiving the effect of different 
rates of planting upon growth of corn in pots. The 
results at different rates of planting without manure are 
here expressed in per cent of the results from one plant 
per pot. Hogue's Yellow Dent corn (1915)* 



Rate of 
planting 


Wt. of 

soil 

(moisture 

-free) 


No. of 

pots 

averaged 


Dsy matter per plant 


Total 
leaf-area 
per plant 


Height of 
stalk 


per pot 


Ear 


Total 


1 
2 
4 
6 


Pounds 
253 
253 
253 
253 


4 
4 
4 
4 


Per cent 
100 
39.7 
15.9 

2.8 


Per cent 

100 

50.8 

26.7 

16.6 


Per cent 

100 

90.7 

67.1 

53.5 


Per cent 

100 

97.5 

86.2 

73.2 



;: Data calculated from Table 44. 



86 Nebraska Agricultural Exp. Station, Research Bui. 13 




Fig. 18 — Normal plants of Hogue's Yellow Dent corn, grown one plant 
per pot, 1915 



Experimental Error in Crop Tests 



87 




Fig. 19 — Plants in the foreground grown six, four and two plants 
per pot 



88 Nebraska Agricultural Exp. Station, Research Bui. 13 




1 



3 



8 



Fig. 2 0- Crop harvested from four pots planted at each of the follow- 
ing rates per pot. Left to right, 1 and 2, one plant per pot; 3 and 
4, two plants per pot; 5 and 6, four plants per pot; 7 and 8, six 
plants per pot. Odd numbers without manure, even numbers with 
manure. (Table 44.) 1914 



Experimental Error in Crop Tests 



89 



tively 16.6, 26.7, and 50.8 per cent as much total dry matter 
as the one-rate, and their yield of ear corn was respectively 
2.8, 15.9, and 39.7 per cent as much per plant. 

An application of 1.55 pounds of manure per pot (Table 
46) increased the yields of total dry matter for the one, two, 
four and six-rates respectively 13.2, 15.3, 18.9, and 29.0 per 
cent. The yields of ear corn were 112.9, 128.3, 100.0, and 
257.0 per cent as large with manure as without manure in 
the one, two, four, and six-rates respectively. 

Table 46 — Summary of data showing the effect of different 
rates of planting upon groivth of corn in pots. The 
results at the different rates of planting with manure are 
here expressed in per cent of the results without manure. 
Hogue's Yellow Dent corn (1915)* 



Rate of 

planting 
per pot 


Wt. m< 
free co 

Soil 


)isture- 
ntents 

Manure 


No. of 

pots 

averaged 


Dry n 
per i 

Ear 


natter 
)lant 

Total 


Total 
leaf- 
area 
per 
plant 


Height 
of stalk 


1 
2 
4 
6 


Pounds 
253 
253 
253 
253 


Pounds 
1.55 
1.55 
1.55 
1.55 


8 
4 
4 
4 


Per cent 
112.9 
128.3 
100.0 
257.0 


Per cent 
113.2 
115.3 
118.9 
129.0 


Per cent 

109.2 

95.3 

110.6 

120.6 


Per cent 

93.5 

93.3 

99.1 

103.3 



:; Data calculated from Table 44. 



STATEMENT OF METHODS IN BULLETINS 

A knowledge of the methods employed in crop testing is 
vital for intelligently evaluating the published results. With- 
out a statement of methods, the reader is obliged to assume 
that reliable methods were employed. Such an assumption 
is not warranted, since many methods used are known to be 
faulty. Not only the experiment station worker but the 
farmer as well should be given an opportunity to know in 
detail how the tests, were made. Increased experimentation 
by farmers has led many of them to be interested in methods. 

The following brief summary table indicates the extent to 
which experiment station bulletins dealing with crop tests 
and published in the United States during the years 1900- 
1914 report details as to methods. A mere statement of re- 
sults is incomplete and does not carry conviction. 



90 Nebraska Agricultural Exp. Station, Research Bui. 13 

Table 47 — Extent to which experiment station bulletins 
report the methods of investigation 



Method details 



Years' duration of tests 

Size of plats 

Shape of plats 

Number of duplicates averaged 

Distribution of duplicates 

Use of check plats 

Number of check plats 

Distribution of check plats .... 

Uniformity of conditions 

Size of pots 

Capacity of pots 

Maturity of crop in pots 



Per cent bulletins* reporting method 
details for 



Variety 

tests 



Per cent 
71 
29 
23 
13 



5 

3 
41 



Fertilizer 

tests 



Per cent 
25 
21 



3 

3 

11 

14 

5 

21 



Cultural 

tests 



Pot 

tests 



Per cent Per cent 



55 



25 
10 
20 
20 
5 
40 
55 
45 
45 



*The total number of bulletins reviewed were: variety tests, 253; fer- 
tilizer tests, 146; cultural tests, 52; pot tests, 20. 



Experimental Error in Crop Tests 91 



BIBLIOGRAPHY 

ALWOOD, W. B., and PRICE, R. H. 

1890. Suggestions Regarding Size of Plats. (Virginia Agricul- 
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BARBER, C. W. 

1914. Note on the Accuracy of Bushel Weight Determinations. 
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1914. Note on the Influence of Shape and Size of Plats in Tests 
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1914. Note on the Influence of Shape and Size of Plots in Tests 
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BULL, C. P. 

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BRIGGS, L. J., and SHANTZ, H. L. 

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CARLETON, M. A. 

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COFFEY, G. N. 

1913. The Purpose and Interpretation of Field Experiments. 
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COLLINS, G. N. 

1914. A More Accurate Method of Comparing First Generation 
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92 Nebraska Agricultural Exp. Station, Research Bui. 13 

CORY, v. L. 

19 08. The Use of Row Plantings to Check Field Plats. (Jour, 
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Experimental Error in Crop Tests 93 

1911. Some Experiments to Estimate Errors in Field Plat Tests. 
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MERCER, W. B., and HALL, O. D. 

1911. Experimental Error of Field Trials. (Jour, of Agricul- 
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MONTGOMERY, E. G. 

1910. Methods for Testing the Seed Value of Light and Heavy 
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1914. A Table for Estimating the Probable Significance of Statis- 
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94 Nebraska Agricultural Exp. Station, Research Bui. 13 

SCHOLZ, H. 

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Experimental Error in Crop Tests 95 

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f t-26-*18 — 4M] 



000 934 594 2 # 



